For the absolute beginner
In Part One of this series, I gave you a brief introduction to PHP, and how it fits into your Web application development environment. I also taught you the basics of PHP variables, and showed you how to add, multiply and concatenate them together.
Now that you know the basics, it's time to focus in on one of PHP's nicer features - its ability to automatically receive user input from a Web form and convert it into PHP variables. If you're used to writing Perl code to retrieve form values in your CGI scripts, PHP's simpler approach is going to make you weep with joy. So get that handkerchief out, and scroll on down.
Forms have always been one of quickest and easiest ways to add interactivity to your Web site. A form allows you to ask customers if they like your products, casual visitors for comments on your site, and pretty girls for their phone numbers. And PHP can simplify the task of processing the data generated from a Web-based form substantially, as this first example demonstrates.
This example contains two scripts, one containing an HTML form (named form.htm) and the other containing the form processing logic (message.php). Here's form.html:
<form action="message.php" method="post">
<label for="msg">Enter your message:</label><input type="text" name="msg">
<input type="submit" value="Send">
</form>
The critical line in this page is the form tag
As you probably already know, the "action" attribute of the form tag specifies the name of the server-side script (message.php in this case) that will process the information entered into the form. The "method" attribute specifies how the information will be passed.
Now for the other half of the puzzle: the message.php script. This script reads the data submitted by the user and "does something with it". Here is message.php:
<?php
// Retrive form data
$input = $_POST['msg'];
// Use it
echo "You said: <em>$input</em>";
?>
When you enter some data into form.htm (let's say "Boo"), and submit it, the form processor message.php will read it and display it to you ("You said: Boo"). Thus, whenever a form is submitted to a PHP script, all variable-value pairs within that form automatically become available for use within the script, through a special PHP container variable: $_POST. You can then access the value of the form variable by using its "name" inside the $_POST container, as I did in the script above.
Obviously, PHP also supports the GET method of form submission. All you need to do is change the "method" attribute to "get", and retrieve values from $_GET instead of $_POST. The $_GET and $_POST variables are actually a special type of PHP animal called an array, which I'll be teaching you about shortly. Don't worry too much about it at the moment, just make sure you're comfortable with retrieving simple values from a form with PHP, and then scroll on down to learn about some more operators that are useful in this context. Try changing message.php to output a message with two input fields (hint: you might want to use the concationation operator we've seen before (.)).
Thus far, the scripts we've discussed have been pretty dumb. All they've done is add numbers and strings, and read back to you the data you typed in yourself - not exactly overwhelming. In order to add some intelligence to your scripts, you need to know how to construct what geeks call a "conditional statement" - a statement which lets your script perform one of a series of possible actions based on the result of a comparison test. And since the basis of a conditional statement is comparison, you first need to know how to compare two variables and determine whether they're identical or different.
You've already seen some of PHP's arithmetic and string operators. However, the language also comes with operators designed specifically to compare two values: the so-called "comparison operators". Here's an example that demonstrates them in action:
<?php
// Define some variables
$mean = 9;
$median = 10;
$mode = 9;
// Less-than operator - returns true if left side is less than right side
// Returns true here
$result = ($mean < $median);
echo "result is $result<br>";
// Greater-than operator - returns true if left side is greater than right side
// Returns false here
$result = ($mean > $median);
echo "result is $result<br>";
// Less-than-or-equal-to operator - returns true if left side is less than or equal to right side
// Returns false here;
$result = ($median <= $mode);
echo "result is $result<br>";
// Greater-than-or-equal-to operator - returns true if left side is greater than or equal to right side
// Returns true here
$result = ($median >= $mode);
echo "result is $result<br>";
// Equality operator - returns true if left side is equal to right side
// Returns true here
$result = ($mean == $mode);
echo "result is $result<br>";
// Not-equal operator - returns true if left side is not equal to right side
// Returns false here
$result = ($mean != $mode);
echo "result is $result<br>";
// Inequality operator - returns true if left side is not equal to right
// Returns false here
$result = ($mean <> $mode);
echo "result is $result<br>";
?>
The result of a comparison test is always Boolean: either true (1) or false (0 - which doesn't print anything). This makes comparison operators an indispensable part of your toolkit, as you can use them in combination with a conditional statement to send a script down any of its multiple action paths.
In addition to the standard equality operator, PHP offers another which allows you to test both for equality and type: the === operator. The following example demonstrates it:
<?php
// Define two variables
$str = '10';
$int = 10;
// Returns true, since both variables contain the same value
$result = ($str == $int);
echo "result is $result<br>";
// Returns false, since the variables are not of the same type even though they have the same value
$result = ($str === $int);
echo "result is $result<br>";
// Returns true, since the variables are the same type and value
$anotherInt = 10;
$result = ($anotherInt === $int);
echo "result is $result";
?>
Read more about PHP's comparison operators at http://www.php.net/manual/en/language.operators.comparison.php.
In addition to the comparison operators I used so liberally above, PHP also provides four logical operators, which are designed to group conditional expressions together. These four operators - logical AND, logical OR, logical XOR and logical NOT - are illustrated in the following example:
<?php
// Define some variables
$auth = 1;
$status = 1;
$role = 4;
// logical AND returns true if all conditions are true
// returns true
$result = (($auth == 1) && ($status != 0));
echo "result is $result<br>";
// logical OR returns true if any condition is true
// returns true
$result = (($status == 1) || ($role <= 2));
echo "result is $result<br>";
// logical NOT returns true if the condition is false and vice-versa
// returns false
$result = !($status == 1);
echo "result is $result<br>";
/* logical XOR returns true if either of two conditions are true, or returns false if both conditions are true */
// returns false
$result = (($status == 1) xor ($auth == 1));
echo "result is $result<br>";
?>
Logical operators play an important role in building conditional statements, as they can be used to link together related conditions simply and elegantly. View more examples of how they can be used at http://www.php.net/manual/en/language.operators.logical.php.
Now that you've learnt all about comparison and logical operators, I can teach you about conditional statements. As noted earlier, a conditional statement allows you to test whether a specific condition is true or false, and perform different actions on the basis of the result. In PHP, the simplest form of conditional statement is the if() statement, which looks something like this:
<?php
if (condition) {
// Do something
}
?>
The argument to if()is a conditional expression, which evaluates to either true or false. If the statement evaluates to true, all PHP code within the curly braces is executed; if it does not, the code within the curly braces is skipped and the lines following the if() construct are executed.
Let me show you how the if() statement works by combining it with a form. In this example, the user is asked to enter his or her age.
<form action="ageist.php" method="post">
<label for="age">Enter your age:<label><input name="age" size="2">
</form>
Depending on whether the entered age is above or below 21, a different message is displayed by the ageist.php script:
<?php
// Retrieve form data
$age = $_POST['age'];
// Check entered value and branch
if ($age >= 21) {
echo 'Come on in, we have alcohol and music awaiting you!';
}
if ($age < 21) {
echo 'You\'re too young for this club, come back when you\'re a little older';
}
?>
Now is a good time to notice the \' there. Remember the strings discussion? Because we're using single quotes to mark where the string starts and finishes, it's broken by that single quote. We can use a backslash there to "escape" the single quote and continue with the string. Remember to play around on your own. A perfectly valid alternative way to do this would be using a double quotes, like:
<?php
echo "You're far too young for this club, come back when you're a little older";
?>
In addition to the if() statement, PHP also offers the if-else construct, used to define a block of code that gets executed when the conditional expression in the if() statement evaluates as false.
The if-else construct looks like this:
<?php
if (condition) {
// Do something
} else {
// Do something else
}
?>
This construct can be used to great effect in the last example: we can combine the two separate if() statements into a single if-else statement.
<?php
// Retrieve form data
$age = $_POST['age'];
// Check entered value and branch
if ($age >= 21) {
echo 'Come on in, we have alcohol and music awaiting you!';
} else {
echo 'You\'re too young for this club, come back when you\'re a little older';
}
?>
PHP also lets you "nest" conditional statements inside each other. For example, this is perfectly valid PHP code:
<?php
if ($day == 'Thursday') {
if ($time == '0800') {
if ($country == 'UK') {
$meal = 'bacon and eggs';
}
}
}
?>
Another, more elegant way to write the above is with a series of logical operators:
<?php
if ($day == 'Thursday' && $time == '0800' && $country == 'UK') {
$meal = 'bacon and eggs';
}
?>
PHP also provides you with a way of handling multiple possibilities: the if-elseif-else construct. A typical if-elseif-else statement block would look like this:
<?php
if (condition) {
// Do something
} else if (condition) {
// Do something
} else if (condition) {
// Do something
} else {
// Do something
}
?>
And here's an example that demonstrates how to use it:
<h2>Today's Special</h2>
<form method="get" action="cooking.php">
<select name="day">
<option value="1">Monday/Wednesday
<option value="2">Tuesday/Thursday
<option value="3">Friday/Sunday
<option value="4">Saturday
</select>
<input type="submit" value="Send">
</form>
As you can see, this is simply a form which allows you to pick a day of the week. The real work is done by the PHP script cooking.php:
<?php
// Get form selection
$day = $_GET['day'];
// Check value and select appropriate item
if ($day == 1) {
$special = 'Chicken in oyster sauce';
} else if ($day == 2) {
$special = 'French onion soup';
} else if ($day == 3) {
$special = 'Pork chops with mashed potatoes and green salad';
} else {
$special = 'Fish and chips';
}
?>
<h2>Today's special is:</h2>
<?php echo $special; ?>
In this case, I've used the if-elseif-else control structure to assign a different menu special to each combination of days. Note that as soon as one of the if() branches within the block is found to be true, PHP will execute the corresponding code, skip the remaining if() statements in the block, and jump immediately to the lines following the entire if-elseif-else block.
It's worth noting that you may either use elseif or else if; they're logically equivelent. Once again, use either, but be consistent.
And that's it for now. To view more examples of conditional statements in action, visit http://www.php.net/manual/en/language.control-structures.php. In Part Three , I'll be bringing you more control structures, more operators and more strange and wacky scripts - so make sure you don't miss it!