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Control Structures - Page 4

January 17, 2002

All the examples of PHP code that we've seen so far have been either simple, one-statement 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 {
  // (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 welcome.php file we created earlier:

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 if-else 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:

if ( $name == "Kevin" ) {
  echo( "Welcome, oh glorious leader!" );
}

The == used in the condition above is the PHP operator that's used to compare two values to see whether they're equal. It's important to remember to type the double-equals, because if you were to use a single equals sign you'd be using the assignment operator discussed above. So, instead of comparing the variable to the designated value, you'd assign a new value to the variable (an operation which, incidentally, evaluates as true). This would not only cause the condition to always be true, but might change the value in the variable you're checking, which could cause all sorts of problems.

Conditions can be more complex than a single comparison for equality. Recall that we modified welcome.php to take 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 (welcome3.php):

if ( "Kevin" == $firstname and "Yank" == $lastname ) {
  echo( "Welcome, oh glorious leader!" );
} else {
  echo( "Welcome to our Web site, $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), they work in PHP as well.

We'll look at more complicated comparisons 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 to repeatedly execute a set of statements. Here's what a while loop looks like:

while ( condition ) {
  // statement(s) to execute over
  // and over as long as condition
  // remains true
}

This 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-and-first), execution jumps immediately to the next 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. This script is available as count10.php in the code archive.

$count = 1;
while ($count <= 10) {
  echo( "$count " );
  $count++;
}

It looks kind of scary, 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 shortcut 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 net result of the code is to output the string "1 2 3 4 5 6 7 8 9 10 ".

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.








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