All the examples of PHP code we’ve seen so far have been either simple, onestatement scripts that output a string of text to the Web page, or series of statements that were to be executed one after the other in order. If you’ve ever written programs in other languages (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 {
// (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 personalized welcome page example we created earlier:
File: welcome6.php (excerpt)
$name = $_REQUEST['name'];
if ($name == 'Kevin') {
echo 'Welcome, oh glorious leader!';
} else {
echo "Welcome to our Website, $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 he appropriate name was entered; 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.
Conditions can be more complex than a single comparison for equality. Recall that our form examples above would receive a first and last name. If we wanted to display a special message only for a particular person, we’d have to check the values of both names:
File: welcome7.php (excerpt)
$firstname = $_REQUEST['firstname'];
$lastname = $_REQUEST['lastname'];
if ($firstname == 'Kevin' and $lastname == 'Yank') {
echo 'Welcome, oh glorious leader!';
} else {
echo "Welcome to my Website, $firstname $lastname!";
}
This condition will be true if and only if $firstname has a value of Kevin and $lastname has a value of Yank. The word and in the above condition makes the whole condition true only if both of the comparisons evaluate to true. Another such operator is or, which makes the whole condition true if one or both of two simple conditions are true. If you’re more familiar with the JavaScript or C forms of these operators (&& and || for and and or respectively), that’s fine—they work in PHP as well.
We’ll look at more complicated conditions as the need arises. For the time being, a general familiarity with the if-else statement is sufficient. Another often-used PHP control structure is the while loop. Where the if-else statement allowed us to choose whether or not to execute a set of statements depending on some condition, the while loop allows us to use a condition to determine how many times we’ll execute a set of statements repeatedly. Here’s what a while loop looks like:
while (condition) {
// statement(s) to execute over
// and over as long as condition
// remains true
}
The while loop works very similarly to an if-else statement without an else clause. The difference arises when the condition is true and the statement(s) are executed. Instead of continuing the execution with the statement that follows the closing brace (}), the condition is checked again. If the condition is still true, then the statement(s) are executed a second time, and a third, and will continue to be executed as long as the condition remains true. The first time the condition evaluates false (whether it’s the first time it’s checked, or the one-hundred-andfirst), execution jumps immediately to the statement that follows the while loop,
after the closing brace.
Loops like these come in handy whenever you’re working with long lists of things (such as jokes stored in a database… hint, hint!), but for now we’ll illustrate with a trivial example: counting to ten.
File: count10.php (excerpt)
$count = 1;
while ($count <= 10) {
echo "$count ";
++$count;
}
It looks a bit frightening, I know, but let me talk you through it line by line. The first line creates a variable called $count and assigns it a value of 1. The second line is the start of a while loop, the condition for which is that the value of $count is less than or equal (<=) to 10. The third and fourth lines make up the body of the while loop, and will be executed over and over, as long as that condition holds true. The third line simply outputs the value of $count, followed by a space.
The fourth line adds one to the value of $count (++$count is a short cut for $count = $count + 1—both will work).
So here’s what happens when this piece of code is executed. The first time the condition is checked, the value of $count is 1, so the condition is definitely true. The value of $count (1) is output, and $count is given a new value of 2. The condition is still true the second time it is checked, so the value (2) is output and a new value (3) is assigned. This process continues, outputting the values 3, 4,
5, 6, 7, 8, 9, and 10. Finally, $count is given a value of 11, and the condition is false, which ends the loop.
The condition in this example used a new operator: <= (less than or equal). Other numerical comparison operators of this type include >= (greater than or equal), < (less than), > (greater than), and != (not equal). That last one also works when comparing text strings, by the way. Another type of loop that is designed specifically to handle examples like that above, in which we’re counting through a series of values until some condition is met, is called a for loop. Here’s what it looks like:
for (initialize; condition; update) {
// statement(s) to execute over
// and over as long as condition
// remains true after each update
}
The initialize statement is executed once at the start of the loop; the condition statement is checked each time through the loop, before the statements in the body are executed; the update statement is executed each time through the loop, but after the statements in the body. Here’s what the “counting to 10” example looks like when implemented with a for loop:
File: count10–for.php (excerpt)
for ($count = 1; $count <= 10; ++$count) {
echo "$count ";
}
As you can see, the statements that initialize and increment the $count variable join the condition on the first line of the for loop. Although, at first glance, the code seems a little more difficult to read, putting all the code that deals with controlling the loop in the same place actually makes it easier to understand once you’re used to the syntax. Many of the examples in this book will use for loops, so you’ll have plenty of opportunity to practice reading them.
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