13. Forms and Control Statements

What We Will Cover

Elucidations

Questions from last class?

Homework Questions?

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13.1: Arrays in PHP

Learner Outcomes

At the end of the lesson the student will be able to:

  • Create an array
  • Populate and reference values from an array
  • Work with array items

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13.1.1: Introduction to Arrays

  • Previously we discussed how PHP handles single-valued data types
  • Now we discuss how PHP handles list of data known as arrays
  • Recall that an array is a collection of data values organized using a single name
  • Each data value has an index to distinguishes it from other values in the array
  • The easiest type of index to work with is a whole number, or integer
  • Each data value (element) of an array is identified by its name and index like this: 
    arrayName[index]
  • For example, we might have an array of the names of the days of the week like the following:

    Element Value
    $dayName[0] "Sunday"
    $dayName[1] "Monday"
    $dayName[2] "Tuesday"
    $dayName[3] "Wednesday"
    $dayName[4] "Thursday"
    $dayName[5] "Friday"
    $dayName[6] "Saturday"

Why Do We Need Arrays?

  • An array is like a little database holding information
  • Once the array is populated with the data, extracting the data is easy
    • A variable name and an index like: $dayName[1]
  • You could use individual variables for all the data
  • However, this get cumbersome quite quickly
  • Imagine keeping track of test scores for an exam
  • Keeping scores for 5 students in 5 different variables is OK, but what about 100 or 1000 students?
    • How would you name all the variables?
    • How would you add the scores up for all the students?
  • Anytime you have more than two or three variables of the same thing, it is usually better to store the data in an array
  • We will see later in this lesson how we can process array data using loops
  • Using a loop, you can do more with arrays than you can with individual variables
  • Also, we will see that PHP uses arrays in many ways such as in form processing

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13.1.2: Creating and Populating an Array

  • Like other PHP variables, arrays do not need to be declared in advance
  • Just type the variable name followed by square brackets
  • You can assign a value to a single array element: 
    $myArray[] = "someValue";
  • After assignment, you can use array element like any other variable: 
    print $myArray[0];
  • By default, arrays indexes start at zero (0)<
  • For example:

    <?php
    // Initializing values
    $myArray[] = "zero";
    $myArray[] = "one";
    $myArray[] = "two";

    // Accessing values
    echo "$myArray[0], $myArray[1], $myArray[2]";
    ?>

  • Produces the output:

    zero, one, two

Using the array() Function

  • You can use the array() function to initialize arrays as well
  • For example:

    <?php
    // Initializing values
    $myArray = array("zero", "one", "two");

    // Accessing values
    echo "$myArray[0], $myArray[1], $myArray[2]";
    ?>

  • Produces the output:

    zero, one, two

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13.1.3: Working with Array Items

  • Individual array elements are accessed using the array name and an index
  • You use the array operator [] to specify the index
  • Arrays indexes can be literal values or a variable:
  • For example:

    <?php
    $dayName     = array("Sunday", "Monday", "Tuesday",
            "Wednesday", "Thursday", "Friday", "Saturday");
    $myIndex = 6;

    echo     "$dayName[0]<br />$dayName[1]<br />";
    echo     $dayName[2]."<br />$dayName[3]<br />";
    echo     "$dayName[4]<br />$dayName[5]<br />";
    echo     $dayName[$myIndex];
    ?>

  • Produces the output:

    Sunday
    Monday
    Tuesday
    Wednesday
    Thursday
    Friday
    Saturday

  • If you wish, you can set start an index with a number other than 0
  • With the following, we set the first index to the number 1
  • The special notation => is used to specify a key value in the array function like: 
    array(1=>"Sunday", ... );
  • Values added to an array without specifying an index use the previously assigned maximum integer index plus one (+1)
  • For instance:

    <?php
    $dayName     = array(1=>"Sunday", "Monday", "Tuesday",
            "Wednesday", "Thursday", "Friday", "Saturday");

    echo     "$dayName[1]<br />$dayName[2]<br />
          $dayName[3]<br />$dayName[4]<br />
          $dayName[5]<br />$dayName[6]<br />"    ;
    echo     $dayName[7];
    ?>

  • Produces the output:

    Sunday
    Monday
    Tuesday
    Wednesday
    Thursday
    Friday
    Saturday

  • Note that if you try to use an index that does not have a value, PHP returns NULL
  • NULL is a special value meaning, "no value assigned"

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13.1.4: Working with Array Functions

  • PHP has many functions that let you interact with and manipulate arrays in various ways
  • For a complete list see the PHP documentation: Array Functions
  • We will look at a few of the more commonly used functions here

Working with Array Length

  • PHP arrays can change size as you use them, unlike some other languages
  • Also, you do not need to define a value for every item
  • Sparse arrays: arrays with several missing or null items
  • To determine the number of items in an array, you can use the count() function: 
    $numItems = count($arrayName);
  • For example:

    <?php
    $dayName     = array("Sunday", "Monday", "Tuesday",
            "Wednesday", "Thursday", "Friday", "Saturday");
    $numItems = count($dayName);
    echo     "count = $numItems";
    ?>

  • Produces the output:

    count = 7

  • Another function that performs the same action as count() is sizeof()

Some Functions to Extract and Insert Array Items

Method Description
array_push() Adds one or more elements to the end of an array
array_pop() Removes and returns the last element of an array
array_unshift() Adds one or more elements to the beginning of an array
array_shift() Removes the first element from an array
array_slice() Extracts items from the middle of an array
array_splice() General purpose method to insert items into or extract items from an array

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13.1.5: Associative Arrays

  • Arrays indices in PHP can be strings rather than numbers
  • Arrays with non-integer indexes are known as associative arrays
  • Sometimes people refer to them as "hash tables" because they are often implemented using hash coding
  • Associative arrays are often used as a lookup table or cross reference between names and values

Creating Associative Arrays

  • You create an associative array like a regular array except you use strings for the index
  • For instance: 
    $course['CIS-132'] = 'Hodges';
$course['CIS-165PH'] = 'Parrish';
$course['DM-173B'] = 'Hodges';
  • You can create associative arrays with the array function as well
  • However, you must specify both the index and the value separated by =>
  • For instance, we can create a cross-reference between month names and number of days in the month: 
    $daysInMonth = array('Jan'=>31, 'Feb'=>28,
  'Mar'=>31, 'Apr'=>30, 'May'=>31, 'Jun'=>30,
  'Jul'=>31, 'Aug'=>31, 'Sep'=>30, 'Oct'=>31,
  'Nov'=>30, 'Dec'=>31);
  • Then we can access an array element like this: 
    $daysInNov = $daysInMonth["Nov"];
  • Note that you can change the value of any array item by assigning a new value: 
    $course['CIS-132'] = 'Parrish';

Example Lookup Table for Days in Months

<?php
$daysInMonth = array('Jan'=>31, 'Feb'=>28,
  'Mar'=>31, 'Apr'=>30, 'May'=>31, 'Jun'=>30,
  'Jul'=>31, 'Aug'=>31, 'Sep'=>30, 'Oct'=>31,
  'Nov'=>30, 'Dec'=>31);
$daysInNov = $daysInMonth["Nov"];
echo "Days in November: $daysInNov";
?>

  • Produces the output:

    Days in November: 30

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13.1.6: Summary

  • In this section, we looked at how to work with arrays
  • An array is a collection of data values organized using a single name
  • Arrays are convenient ways to process a list of data
  • Like other PHP variables, arrays do not need to be declared in advance
  • Just type the variable name followed by square brackets
  • You can assign a value to a single array element: 
    $myArray[] = "someValue";
  • After assignment, you can use array element like any other variable: 
    print $myArray[0];
  • An array function uses more compact code to initialize arrays: 
    $myArray = array("zero", "one", "two");
  • You can declare arrays in a form for processing in your PHP scripts
  • Place square brackets '[]' after the element's name
  • Array indexes in PHP can be strings rather than numbers
  • Allows cross-referencing, among other uses: 
    $months = array('Jan'=>31, 'Feb'=>28, 'Mar'=>31,
  'Apr'=>30, 'May'=>31, 'Jun'=>30, 'Jul'=>31,
  'Aug'=>31, 'Sep'=>30, 'Oct'=>31, 'Nov'=>30,
  'Dec'=>31);

Check Yourself

  1. What is an array?
  2. What are the two ways you can declare and initialize an array?
  3. What statement would you use to create an array named $dayNames?
  4. What statements would you use to create an array named $dayNames and initialize it with the abbreviations for the days of the week, starting with "Sun" and going through "Sat"?
  5. What expression would you use to return the third value from the array $dayNames?

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Activity 13.1

Take one minute to read over the following Quick Quiz questions. We will discuss the questions in one minute.

Quick Quiz

  1. In an array, each individual data value has a number or that distinguishes it from other values in the array.
  2. With the following code, you are creating an array with __ elements. 
    $dayName = array(1=>"Sunday", "Monday", "Tuesday",
    "Wednesday", "Thursday", "Friday", "Saturday");
    1. 1
    2. 6
    3. 7
    4. 8

  3. The syntax count($arrayName) where $arrayName is the name of the array, is a function that counts the number of elements in an array.

    True
    False

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13.2: Forms and PHP

Learner Outcomes

At the end of the lesson the student will be able to:

  • Pass data from HTML forms to PHP scripts
  • Use PHP functions to examine the form data
  • Save data in PHP variables for processing
  • Acquire data from multiple-valued form elements

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13.2.1: Coding Forms for PHP

  • Recall that you construct a form using the syntax: 
    <form attributes>
    page elements
</form>
  • Where the attributes control how the form is processed
  • Inside the form tags you include various control elements and other page elements such as tables, paragraphs and headings
  • Some important form attributes for PHP include:
    • action: specifies the URL where the form sends the data for processing
    • method: specifies how to send the information
      • The get method appends the information to the URL
      • The post method sends the data within the body of the request
  • For example: 
    <form action="formecho.php" method="get">
</form>

Input Elements

  • Most form controls in which users enter data are marked with an input element
  • Syntax of an input element with attributes required for PHP: 
    <input type="type" name="name" />
  • Where type is one of the 10 types and name is the identifier for the element
  • The name attribute is required for processing the data using PHP
  • HTML has 10 different input types as shown in the following table
  • Each of them has various attributes that specify their behavior
  • You can view the source to see how some of the attributes are used

Input Types

Type Description Example
type="button" Display a button to perform an action from a script
type="checkbox" Display a check box
type="file" Display a control to locate and select a file
type="hidden" Create a hidden field not viewable in the form
type="image" Display an inline image that can be clicked to submit data
type="password" Display a text field that hides text entered by the user
type="radio" Display an option (radio) button One Two
type="reset" Display a button to clear the form data when clicked
type="submit" Display a button to submit the form data when clicked
type="text" Display a text field that displays text entered by the user

Other Form Elements

  • Forms have other ways for users to enter data
  • These include the text area, which has the syntax: 
    <textarea name="name" rows="value" cols="value">
default text
</textarea>
  • For example, this code produces the text area that follows:

    <textarea name="comments" rows="3" cols="50">
    Your comments here
    </textarea>

  • The name attribute is required for processing the data using PHP
  • Another commonly used form element is the selection list, which we discuss later

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13.2.2: Acquiring Form Data in PHP Scripts

  • PHP makes it easy to receive input data from forms
  • For instance, given the following input element: 
    <input type="text" name="fname">
  • We use $_REQUEST with square brackets and the element name like this: 
    $firstName = $_REQUEST["fname"];
print "firstName: $firstName";
  • If you only want to accept data passed using the GET method, use $_GET instead: 
    $firstName = $_GET["fname"];
print "firstName: $firstName";
  • If you only want to accept data passed using the POST method, use $_POST instead: 
    $firstName = $_POST["fname"];
print "firstName: $firstName";
  • These arrays are known as "autoglobals" or "superglobals" because they are automatically global throughout your PHP page

Superglobal Arrays Used with Forms

Array Description
$_GET Access to variables passed via the HTTP GET method.
$_POST Access to variables passed via the HTTP POST method.
$_REQUEST Access to variables passed via both the HTTP GET and POST methods.

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13.2.3: Examining Form Data

  • Let us look at an example of form processing using PHP
  • In the form below we include the important attributes for PHP:
    • action: specifies the URL where the form sends the data for processing
    • method: specifies how to send the information
    • name: specifies the name for each form element
    • value: specifies the value for some types of form elements
  • Following form is the script for reading the form data
  • To read the data, the script uses the $_REQUEST array
  • To help debug forms and other data, PHP provides two functions you can use:
    • print_r(): prints a compact form of data about a variable
    • var_dump(): prints detailed information about a variable
  • The script displays the $_REQUEST array using both print_r() and var_dump()
  • The data from the functions is displayed inside of preformatted text elements
  • The pre-tags make the data more readable in a Web page
  • Following these functions are data extracted into single variables
  • Also, the script displays the variables
  • You can view the form here: Simple Input Form

Example Form for PHP Processing

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
  "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Simple Input Form</title>
</head>

<body onload='document.forms[0].elements[0].focus();'>
<h2>Simple Input Form</h2>
<form method="get" action="formecho.php">
<table border="0" cellspacing="0" cellpadding="5">

<tr>
<td>First Name:</td>
<td><input type="text" name="fname" /></td>
</tr>

<tr>
<td>Last Name:</td>
<td><input type="text" name="lname" /></td>
</tr>

<tr>
<td class="label">Shipping Option:</td>
<td>
<input type="radio" name="state" id="s1" value="CA" />
<label for="s1">California</label><br />
<input type="radio" name="state" id="s2" value="OR" />
<label for="s2">Oregon</label><br />
<input type="radio" name="state" id="s3" value="WA" />
<label for="s3">Washington</label>
</td>
</tr>

<tr>
<td class="label">&nbsp;</td>
<td>
<input type="submit" name="submit" value="Continue">
</td>
</tr>

</table>
</form>

</body>
</html>

Example Script Reading the Form Data

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
  "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Form Echo</title>
</head>

<body>
<h2>Form Echo</h2>
<h3>Data Using <code>print_r()</code></h3>
<pre><?php print_r($_REQUEST); ?></pre>

<h3>Data Using <code>var_dump()</code></h3>
<pre><?php var_dump($_REQUEST); ?></pre>

<h3>Data into Single Variables</h3>
<?php
$firstName = $_REQUEST["fname"];
$lastName = $_REQUEST["lname"];
$state = $_REQUEST["state"];
$submit = $_REQUEST["submit"];

print "firstName: $firstName<br />";
print "lastName: $lastName<br />";
print "state: $state<br />";
print "submit: $submit";
?>
</pre>
</body>
</html>

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13.2.4: Working with Selection Lists

  • Recall that a selection list is a list box from which a user selects a value or values
  • The syntax for the select and option elements is: 
    <select id="id" name="name">
    <option value="value1">Item1</option>
    <option value="value2">Item2</option>
    ...
</select>
  • For example, to code a selection list that returns a single value: 
    <select id="state" name="state">
  <option value="CA">California</option>
  <option value="OR" selected="selected">Oregon</option>
  <option value="WA">Washington</option>
</select>
  • Which produces the output:

  • A selection list that selects a single option is read like an input element by PHP
  • for instance, to read the option form the above selection list: 
    $state = $_REQUEST["state"];
print "state: $state";

Reading Multiple Values

  • Selection lists can return more than one value by including the multiple attribute
  • For example: 
    <select id="state2" name="state" multiple>
  <option value="CA">California</option>
  <option value="OR">Oregon</option>
  <option value="WA">Washington</option>
</select>
  • Which produces the output:

  • To select multiple values, you hold down the Ctrl key while selecting with the mouse
  • Normally, PHP only reads a single value from the potentially multiple selections
  • To read multiple values, you need to use square brackets after the name of the select list
  • For the above example: 
    <select id="state" name="state[]" multiple>
  • This tells PHP to store the multiple values in an array
  • You can see this in the following example
  • Also, you can view the form here: Form with Select List
  • Note that we need some way to process a variable number of items in the array
  • We will discuss how to do this using loops later in the lesson

Form with Select List for PHP Processing

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
  "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Form with Select List</title>
</head>

<body onload='document.forms[0].elements[0].focus();'>
<h2>Form with Select List</h2>
<form method="get" action="formecho.php">
<table border="0" cellspacing="0" cellpadding="5">

<tr>
<td>First Name:</td>
<td><input type="text" name="fname" /></td>
</tr>

<tr>
<td>Last Name:</td>
<td><input type="text" name="lname" /></td>
</tr>

<tr>
<td class="label">State:</td>
<td>
<select name="state[]" multiple>
<option value="CA">California</option>
<option value="OR">Oregon</option>
<option value="WA">Washington</option>
</select>
</td>
</tr>

<tr>
<td class="label">&nbsp;</td>
<td>
<input type="submit" name="submit" value="Continue">
</td>
</tr>

</table>
</form>

</body>
</html>

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13.2.5: Summary

  • In this section we looked at how you can use forms with PHP
  • Each form control element must have a name assigned to be read by PHP
  • PHP can read the form data using the superglobal array $_REQUEST, $_POST or $_GET
  • For instance, given the following input element: 
    <input type="text" name="fname">
  • We use $_REQUEST with square brackets and the element name like this: 
    $firstName = $_REQUEST["fname"];
print "firstName: $firstName";
  • You can view the data passed from a form using either:
    • print_r(): prints a compact form of data about a variable
    • var_dump(): prints detailed information about a variable
  • For example: 
    print_r($_REQUEST);
  • Some form elements, like selection lists, can return multiple values
  • To let PHP know that the form element can have multiple values, you must put square brackets after the name
  • For example: 
    <select name="state[]" multiple>

Check Yourself

  1. What attribute must a form control element contain to be read by PHP?
  2. What PHP code can you use to read a form element named "lname"?
  3. What PHP functions can you use to view all the values of a form?
  4. When a form control element can return multiple values, what special symbols must you include after the name?

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Activity 13.2

Take one minute to read over the Review Questions in the textbook. We will discuss the questions as time permits.

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13.3: Relationships, Truth and Conditions

Learner Outcomes

At the end of the lesson the student will be able to:

  • Use conditional statements to compare numerical and string data values
  • Use conditional statements to test form values

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13.3.1: Relationship Values and Truth

  • In addition to arithmetical operations, all computers can compare numbers
  • Many decisions can be reduced to choosing between two numbers
  • Comparing numbers can make computers seem "intelligent"
  • Expressions that compare two numbers are called relational expressions
    • Also called conditions
  • Simple relational expressions have one relational operator comparing two values:

    relational expression

  • A relational expression always evaluates to either true or false
  • You can see the relational operators in the following table
  • Note that operators === and !== include the type of the data when making the comparison

Relational Operators

Math Name PHP Example Result
= Equal to == or
===		 5 == 10
2 == 2		 false
true
≠ Not equal to != or
!==		 5 != 10
2 != 2		 true
false
< Less than < 5 < 10
5 < 5
5 < 2		 true
false
false
≤ Less than or equal to <= 5 <= 10
5 <= 5
5 <= 2		 true
true
false
> Greater than > 5 > 10
5 > 5
5 > 2		 false
false
true
≥ Greater than or equal to >= 5 >= 10
5 >= 5
5 >= 2		 false
true
true

What is Truth?

  • "What is truth?" may seem more appropriate for philosophy than programming
  • However, in programming we have to know how the computer interprets truth
  • PHP interprets the following as false and every other value as true:
    • boolean FALSE
    • integer 0 (zero)
    • float 0.0 (zero)
    • empty strings "", and the string "0"
    • arrays with zero elements (not covered yet)
    • objects with zero elements (not covered yet)
    • special type NULL (such as variables not assigned a value)
  • Note that a NULL character (\0) does not display anything in a browser
  • To view information about variables, you can use var_dump()
  • Thus, to see the truth of a relational expression, you can use something like: 
    var_dump(5 == 10);

More Information

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13.3.2: Using the if-else Statement

  • The if-else statement lets us choose between two alternatives based on a condition
  • A condition is some state upon which the execution of some statements depends
  • The condition must evaluate to either true or false
  • If a condition evaluates to true
    • then do this
  • Otherwise it is false
    • so do something else
  • The syntax of the if-else statement: 
    if (condition) {
    statements1;
} else {
    statements2;
}
  • As an example, we can code an if-else statement like this: 
    if ($_REQUEST["guess"] == 7) {
    print "<p>*** Correct! ***</p>\n";
} else {
    print "<p>Sorry, that is not correct.</p>\n";
}
  • For clarity, if and else are written on different lines than the nested statements
  • Also, the nested statements are indented
  • For an example using an if-else statement, we will write a simple guessing game application
  • We collect a guess from the player with the simple form shown below
  • We then evaluate the form with the test script
  • You can play the game yourself by clicking: Guessing Game

Form Collecting the Players Guess

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
  "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Guessing Game</title>
</head>

<body onload='document.forms[0].elements[0].focus();'>
<h2>Guessing Game</h2>
<p>I'm thinking of a number between 1 and 10.<p>
<p>Can you guess it?</p>

<form action="guesstest.php" method="get">
<input type="text" name="guess"><br /><br />
<input type="submit" name="submit" value="Check Guess">
<input type="reset" value="Reset">
</form>

</body>
</html>

Script Testing the Form Data

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
  "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Guessing Game Response</title>
</head>
<body>
<?php
if ($_REQUEST["guess"] == 7) {
    print "<p>*** Correct! ***</p>\n";
    print '<a href="guessform.html">Play again</a>';
} else {
    print "<p>Sorry, that is not correct.</p>\n";
    print '<a href="guessform.html">Guess again</a>';
}
?>
</body>
</html>

Omitting the else Clause

  • Note that the else clause can be omitted entirely: 
    if (condition) {
   // execute statements only if true
}
  • What would happen if we omitted the else clause in the guessing game test script?

More Information

  • if: the PHP Manual
  • else: the PHP Manual

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13.3.3: Using the elseif Statement

  • You use an elseif clause when you have a list of values to consider
  • The elseif is the same as an if statement followed by an else
  • An elseif lets you specify multiple test conditions
  • Only executes when previous conditions are false: 
    if (condition1 == true) {
    // execute statements only if condition1 is true
} elseif (condition2 == true) {
    // execute statements only if condition2 is true
    // and condition1 is false
} else {
    // execute statements if nothing preceding is true
}
  • Tests multiple conditions until the first true condition is found
  • If a true condition is found, executes code within the block
  • Then the computer skips the remainder of the if-elseif-else chain
  • You can end with an else clause for when no conditions are true

For Example

  • We rewrite our previous example to use an if-elseif-else structure: 
    if ($_REQUEST["guess"] == 7) {
    print "<p>*** Correct! ***</p>\n";
} elseif ($_REQUEST["guess"] < 7) {
    print "<p>Sorry, that is too low.</p>\n";
} else {
    print "<p>Sorry, that is too high.</p>\n";
}
  • Note the arrangement of elseif statements
  • What is displayed if the user enters 5? Why?
  • What is displayed if the user enters 9? Why?

More Information

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13.3.4: Working with Logical Operators

  • Sometimes you want to consider several relational expressions at once
  • For this we can use logical operators
  • Use ! (NOT) operator to reverse value of expression
  • Use and to AND two or more conditions
    • Also, you can spell AND as: &&
  • Use or to OR two or more conditions
    • Also, you can spell OR as: ||
  • and operator returns true only if expressions on both sides are true
  • or operator returns true if either expression is true
  • You can see examples of these conditions in the following tables
! Operator
If expr is... Then ! expr is... Example Result
true false !true false
false true !(5 < 2) true

and (&&) Operator
If expr1 is... And expr2 is... Then expr1 and expr2 is... Example Result
true true true 5 < 10 and 5 > 2 true
true false false 5 < 10 and 5 < 2 false
false true false 5 > 10 and 5 > 2 false
false false false 5 > 10 and 5 < 2 false

or (||) Operator
If expr1 is... || expr2 is... Then expr1 or expr2 is... Example Result
true true true 5 < 10 or 5 > 2 true
true false true 5 < 10 or 5 < 2 true
false true true 5 > 10 or 5 > 2 true
false false false 5 > 10 or 5 < 2 false

Testing a Range of Values

  • Sometimes we want to verify that users enter correct values, such as a number between 1 and 10
  • You can use a logical or to check the value of two fields at once: 
    if ($num < 1 or $num > 10) {
  print "Error: enter a number between 1 and 10";
}
  • An alternate way of checking uses the NOT operator with the AND operator: 
    if (!($num >= 1 and $num <= 10)) {
  print "Error: enter a number between 1 and 10";
}
  • Tip: construct a truth table to verify your logic

Check Yourself

  • What is displayed by running the following code? 
    if ($bb or !$bb) {
    print "That is the question!";
}

More Information

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13.3.5: Comparing Strings

  • PHP stores string values using the ASCII code
    • Acronym for American Standard Code for Information Interchange
    • Pronounced ("ask-ee")
  • ASCII provides a standard, numerical way to represent characters
  • Every letter, number, and symbol is translated into a code number
  • Digits are stored in order from '0' to '9':
    • Digit '0' is code value 48
    • Digit '1' is code value 49
    • And so on to '9', which is code value 57
  • Uppercase letters of the alphabet are stored in order from 'A' to 'Z':
    • Letter 'A' is code value 65
    • Letter 'B' is code value 66
    • Up to 'Z' which is code value 90
  • Similarly, lowercase letters are stored in order from 'a' to 'z':
    • Letter 'a' is code value 97
    • Letter 'b' is code value 98
    • Up to 'z' which is code value 122
  • Also, a space is code value 32, which is less than all letters and digits

Comparing Codes

  • Notice that digits come before uppercase letters
  • Also, uppercase letters come before lowercase letters
  • Since all letters and digits are numbers, you can compare strings just like numbers
  • PHP lets you use all the relational operators to compare string values:
    ==, !=, <, >, <= and >=
  • These operators produce results like you would expect when comparing words alphabetically
  • The following example shows the results of comparing two words
  • You can try the form by clicking: String Input

Form Collecting the Strings for Comparison

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
  "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>String Input</title>
</head>

<body onload='document.forms[0].elements[0].focus();'>
<h2>String Input</h2>
<p>Enter two strings and I will compare them.<p>

<form action="stringcompare.php" method="get">
<table border="0" cellspacing="0" cellpadding="6">
<tr><td>
<label for="s1">First string:</label>
</td>
<td>
<input type="text" name="s1" id="s1">
</td></tr>
<tr><td>
<label for="s2">Second string:</label>
</td>
<td>
<input type="text" name="s2" id="s2">
</td></tr>
<tr><td>&nbsp;</td>
<td>
<input type="submit" name="submit" value="Compare">
<input type="reset" value="Reset">
</td></tr>
</table>
</form>

</body>
</html>

Script Comparing the Strings

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
  "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>String Comparison</title>
</head>
<body>
<?php
$s1 = $_REQUEST["s1"];
$s2 = $_REQUEST["s2"];
if ($s1 == $s2) {
    print "<p>The strings are equal</p>\n";
} elseif ($s1 < $s2) {
    print "<p>'$s1' is less than '$s2'</p>\n";
} else {
    print "<p>'$s1' is greated than '$s2'</p>\n";
}
?>
</body>
</html>

More Information

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13.3.6: Summary

  • Many decisions can be reduced to choosing between two numbers
  • Comparing numbers can make computers seem "intelligent"
  • To compare numbers, you use relational operators
  • PHP relational operators include: 
    ==  ===  !=  !==  <  <=  >  >=  <=  >=
  • Similarly, you can compare letters since letters are stored as numbers
  • Relational expressions always evaluate to either true or false
  • To make choices in your scripts, use one of the following statements: 
    if
if-else
if elseif elseif ... else
  • When you need to compare several conditions, you use logical operators: 
    !   and   or
  • For example: 
    if (($num < 1) or ($num > 10) {
  print "Error: enter a number between 1 and 10";
}

Check Yourself

  1. Which PHP values evaluate to false and which evaluate to true?
  2. Why can you compare character data using a relational expression?
  3. When does an and (&&) of two or more conditions evaluate to true?
  4. When does an or (||) of two or more conditions evaluate to false?
  5. What is the effect of the NOT (!) operator?

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Activity 13.3

Take one minute to review the Check Yourself questions. We will discuss the questions as time permits.

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13.4: Using Loops to Repeat Statements

Learner Outcomes

At the end of the lesson the student will be able to:

  • Work with for loops
  • Work with while loops
  • Loop through the contents of an array

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13.4.1: About Programming Loops

  • Sometimes we want to repeat certain statements in PHP
  • For example, consider the code to produce a selection list:

    <select id="weekday" name="weekday">
      <option value="Sunday">Sunday</option>
      <option value="Monday">Monday</option>
      <option value="Tuesday">Tuesday</option>
      <option value="Wednesday">Wednesday</option>
      <option value="Thursday">Thursday</option>
      <option value="Friday">Friday</option>
      <option value="Saturday">Saturday</option>
    </select>

  • Which produces the form control:

  • Note that this code contains a lot of repeated text
  • It may not seem so bad for only seven items, but it can become cumbersome with 50, 100 or more items
  • Programmers deal with this type of situation using loops
  • A loop is a set of statements that are executed repeatedly
  • PHP has 3 general-purpose looping statements: 
    while
do-while
for
  • We will start with the while loop statement

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13.4.2: Using the while and do-while Loops

  • The while loop is the simplest loop statement with the syntax: 
    while (condition) {
    statements to repeat
}
  • The statements inside the loop execute as long as the condition evaluates to true
  • When the loop finishes, the flow of control returns to the while statement
  • You can see the flow in the following diagram
  • Note that if the loop condition is initially false then the loop body never executes

Diagram of while Loop Operation

While flow chart

Using the while Loop

  • You can use a while loop to count by adding a counter variable
  • For example:

    <?php
          //Initialization
          $count = 0;
      echo     "Before loop: count = $count" ;
      while (    $count < 5) { //loop condition
          //loop body
              $count = $count + 1;
          echo     "<br>count = $count\n";
      }
      echo     "<br>After loop: count = $count" ;
    ?>

  • Produces the output:

    Before loop: count = 0
    count = 1
    count = 2
    count = 3
    count = 4
    count = 5
    After loop: count = 5

About the do-while Loop

  • A variation on the while loop is the do-while loop
  • The difference between the two loops is where the condition is checked
  • In the do-while loop, the condition is checked at the end of the loop block
  • Syntax: 
    do {
    statements to repeat
} while (condition);
  • The loop continues as long as the condition evaluates to true
  • By checking the condition at the end of the loop, the statements to repeat are guaranteed to execute at least one time

More Information

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13.4.3: Using the for Loop

  • The for loop provides a compact alternative to the while loop
  • Syntax: 
    for (initialization; condition; iteration) {
   statements to repeat
}
  • initialization: a statement that defines the initial conditions of the loop
  • condition: an expression that determines when to continue (or end) the loop
  • iteration: a statement evaluated at the end of each loop iteration
  • The statements to repeat are enclosed in the curly brackets

Execution Sequence

  1. When the for loop is reached, execute the initialization statement
  2. Check if condition is true
    • if true then continue with Step 3
    • Otherwise, continue with Step 6
  3. Execute the statements inside the curly braces
  4. When end of loop body is reached, execute the iteration statement
  5. Return to Step 2
  6. Loop is finished: continue with statements after the loop

Example Using the for Loop

  • A common use of a for loop is to count
  • You use a counter variable to track number of times to run the block of statements
  • For example:

    <?php
        $max     = 5;
        for (    $i = 0; $i < $max; $i = $i + 1) {
            echo     "i = $i <br>";
        }
        echo     "After loop i = $i" ;
    ?>

  • Produces the output:

    i = 0
    i = 1
    i = 2
    i = 3
    i = 4
    After loop i = 5

  • You can use a counting loop like this to repeat a block of code a certain number of times
  • Note that the iteration statement is not limited to increasing the counter by one

Check Yourself

  • Lets check our understanding of the for loop with the following code
  • Write down what you think the program will display before you check the answer
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
   "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
  <title>Testing the PHP for Loop</title>
</head>

<body>
<?php
    print "First loop:<br />";
    for ($count = 0; $count < 5; $count++) {
        print "$count<br />";
    }
    print "Second loop:<br />";
    for ($count = 5; $count > 0; $count--) {
        print "$count<br />";
    }
    print "Third loop:<br />";
    for ($i = 0; $i <= 360; $i += 60) {
        print "$i<br />";
    }
    print "Fourth loop:<br />";
    for ($i = 1; $i <= 64; $i *= 2) {
        print "$i<br />";
    }
?>
</body>
</html>
Check answer

About those Curly Braces

  • Technically, the a loop statement affects only the single statement that follows
  • We use curly braces to make that one statement into a block of statements
  • This allows us to put any number of statements within the block
  • Curly braces are not always required, but the best practice is to always include them

More Information

  • for: the PHP Manual

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13.4.4: Using Arrays and Loops to Create Selection Lists

  • A for loop is often used to iterate (Link to pronunciation) through every value in an array
  • The general structure for accessing each value in array: 
    $length = count($arrayName);
for ($index = 0; $index < $length; $index++) {
   statements involving arrayName[$index]
}
  • Where $arrayName is the array with the values and $index is the counter variable
  • Since the last item in the array has an index of one less than the array length, the loop condition use a less-than operator

Displaying a Selection List

  • Using this structure, let us use arrays and loops to create a selection list
  • First we create an array containing the data we want to display: 
    $dayName = array("Sunday", "Monday", "Tuesday",
    "Wednesday", "Thursday", "Friday", "Saturday");
  • Then we loop through the values of the array and print each array value in an option tag: 
    for ($i = 0; $i < count($dayName); $i++) {
    print "<option value=\"$dayName[$i]\">";
    print "$dayName[$i]</option>\n";
}
  • Finally, we place the PHP code between the opening and closing select tags
  • You can see the complete code for this in the following example

Code Creating a Selection List

<select id="weekday1" name="weekday">
<?php
    $dayName = array("Sunday", "Monday", "Tuesday",
        "Wednesday", "Thursday", "Friday", "Saturday");
    for ($i = 0; $i < count($dayName); $i++) {
        print "<option value=\"$dayName[$i]\">";
        print "$dayName[$i]</option>\n";
    }
?>
</select>

  • Produces the output:

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13.4.5: Using the foreach Loop

  • Since loops are used so often with arrays, PHP has a special loop optimized for use with arrays: foreach
  • Syntax: 
    foreach ($arrayName as $item) {
    statements to repeat
}
  • Where $arrayName is the array and $item is the value extracted from the array on each iteration
  • For each iteration of the array, the loop assigns the next value in the list to $item
  • Then you can use $item inside the curly-braces of the array

Example Using foreach

  • Lets revisit our selection list example using the foreach loop

    <select id="weekday2" name="weekday">
    <?php
        $dayName     = array("Sunday", "Monday", "Tuesday",
                "Wednesday", "Thursday", "Friday", "Saturday");
        foreach (    $dayName as $day) {
            print     "<option value=\"$day\">";
            print     "$day</option>\n";
        }
    ?>
    </select>

  • Produces the output:

  • Notice how the loop code is simplified by removing the counting variable

More Information

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13.4.6: Summary

  • Loops are used to repeat sections of code
  • The while loop is the simplest loop statement with the syntax: 
    while (condition) {
    statements to repeat
}
  • A variation on the while loop is the do-while loop: 
    do {
    statements to repeat
} while (condition);
  • By checking the condition at the end of the loop, the statements to repeat are guaranteed to execute at least one time
  • A more compact repetition statement is the for loop: 
    for (initialization; condition; iteration) {
   statements to repeat
}
  • Loops are often used as a way to iterate through array items
  • PHP has a special type of loop for use with arrays named foreach
  • The syntax of the foreach loop is: 
    foreach ($arrayName as $item) {
    statements to repeat
}
  • For each iteration of the array, the loop assigns the next value in the list to $item

Check Yourself

  1. What is a loop?
  2. What is the purpose of a loop test condition?
  3. What is the code for a for statement that uses a counter variable named counter that starts with the value 0 and continues up to and including 100 in increments of 10?
  4. What loop statement would you code to create a selection list with the values and option text set to the items in an array?

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Activity 13.4

Take one minute to review the Check Yourself questions. We will discuss the questions as time permits.

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Wrap Up

Due Next:
Final Project Report and Presentation (12/11/06)

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Last Updated: November 27 2006 @20:12:53