Incrementing and Decrementing | Spring Into PHP 5

A common thing to do in PHP scripts, especially in the loops we'll see later in this chapter, is to increment the value in a variable by adding one or to decrement the value in a variable by subtracting one. In PHP, the incrementing operator is ++, and the decrementing operator is --.

For example, if $bananas holds 0, then using the ++ operator like this: $bananas++ leaves a result of 1 in $bananas. If $apples holds 11, then $apples-- leaves 10 in $apples.

You can use ++ and -- before or after a variable name, such as ++$bananas (this is using it as a prefix operator) or $bananas++ (this is using it as a postfix operator). If you use ++$bananas, the value in $bananas is incremented, and that incremented value is then used in the rest of the statement. If you use $bananas++, then the original value in $bananas is used in the rest of the statement, and then the value in $bananas is incremented. Here's an overview of the difference:

`++$value`	Pre-increment	Increments `$value` by one, then returns `$value`.
`$value++`	Post-increment	Returns `$value`, then increments `$value` by one.
`--$value`	Pre-decrement	Decrements `$value` by one, then returns `$value`.
`$value--`	Post-decrement	Returns `$value`, then decrements `$value` by one.

In other words, if you use ++$bananas, the value in $bananas is incremented before that value is passed on for use in the rest of the current statement. If you use $bananas++, the current value in $bananas is passed on for use in the rest of the statement, and only then is the value incremented.

You can see this difference in Example 2-2, phpincrementing.php, which works through the various possibilities, using ++ and -- as both a prefix and postfix operator.

Example 2-2. Incrementing and Decrementing, phpincrementing.php

 <HTML>     <HEAD>        <TITLE>            Incrementing and Decrementing        </TITLE>     </HEAD>     <BODY>         <H1>             Incrementing and Decrementing         </H1> <?php     $a = $b = $c = $d = 1;     echo "\$a++ gives ", $a++, "<BR>";     echo "++\$b gives ", ++$b, "<BR>";     echo "\$c- - gives ", $c--, "<BR>";     echo "- -\$d gives ", --$d, "<BR>"; ?>     </BODY> </HTML>

The results appear in Figure 2-2. As you can see, it makes a difference whether you use ++ and -- as prefix operators or postfix operators.

Figure 2-2. Using the increment and decrement operators.

If you just want to make sure you increment or decrement the value in a variable without worrying about whether the value will be incremented or decremented before being used in the rest of the statement, use ++ and -- as prefix operators.