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Control Structures
All the examples of PHP code that we've seen so far have been either simple,
onestatement
scripts that output a string of text to the Web page, or have been series
of statements that were to be executed one after the other in order. If you've
ever
written programs in any other languages (be they JavaScript, C, or BASIC) you
already know that practical programs are rarely so simple.
PHP, just like any other programming language, provides facilities that allow
us
to affect the flow of control in a script. That is, the language contains special
statements that permit you to deviate from the one-after-another execution order
that has dominated our examples so far. Such statements are called control
structures. Don't get it? Don't worry! A few examples will illustrate perfectly.
The most basic, and most often-used, control structure is the if-else statement.
Here's what it looks like:
if ( condition ) {
// Statement(s) to be executed if
// condition is true.
} else {
3$_REQUEST is not available in versions of PHP prior to PHP 4.1.
59
Control Structures
// (Optional) Statement(s) to be
// executed if condition is false.
}
This control structure lets us tell PHP to execute one set of statements or
another,
depending on whether some condition is true or false. If you'll indulge my vanity
for a moment, here's an example that shows a twist on the welcome1.php file
we
created earlier:
$name = $_REQUEST['name'];
if ( $name == 'Kevin' ) {
echo( 'Welcome, oh glorious leader!' );
} else {
echo( "Welcome, $name!" );
}
Now, if the name variable passed to the page has a value of Kevin, a special
message
will be displayed. Otherwise, the normal message will be displayed and will
contain the name that the user entered.
As indicated in the code structure above, the else clause (that part of the
ifelse
statement that says what to do if the condition is false) is optional. Let's
say you wanted to display the special message above only if the appropriate
name
was entered, but otherwise, you didn't want to display any message. Here's how
the code would look:
$name = $_REQUEST['name'];
if ( $name == 'Kevin' ) {
echo( 'Welcome, oh glorious leader!' );
}
The == used in the condition above is the PHP equal-to operator that's used
to
compare two values to see whether they're equal.
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