PHP supports C-style pre- and post-increment and decrement operators.
Note: The increment/decrement operators only affect numbers and strings. Arrays, objects, booleans and resources are not affected. Decrementing
nullvalues has no effect too, but incrementing them results in1.
| Example | Name | Effect |
|---|---|---|
| ++$a | Pre-increment | Increments $a by one, then returns $a. |
| $a++ | Post-increment | Returns $a, then increments $a by one. |
| --$a | Pre-decrement | Decrements $a by one, then returns $a. |
| $a-- | Post-decrement | Returns $a, then decrements $a by one. |
Here's a simple example script:
<?php
echo "<h3>Postincrement</h3>";
$a = 5;
echo "Should be 5: " . $a++ . "<br />\n";
echo "Should be 6: " . $a . "<br />\n";
echo "<h3>Preincrement</h3>";
$a = 5;
echo "Should be 6: " . ++$a . "<br />\n";
echo "Should be 6: " . $a . "<br />\n";
echo "<h3>Postdecrement</h3>";
$a = 5;
echo "Should be 5: " . $a-- . "<br />\n";
echo "Should be 4: " . $a . "<br />\n";
echo "<h3>Predecrement</h3>";
$a = 5;
echo "Should be 4: " . --$a . "<br />\n";
echo "Should be 4: " . $a . "<br />\n";
?>
PHP follows Perl's convention when dealing with arithmetic operations
on character variables and not C's. For example, in PHP and Perl
$a = 'Z'; $a++; turns $a into 'AA', while in C
a = 'Z'; a++; turns a into '['
(ASCII value of 'Z' is 90, ASCII value of '[' is 91).
Note that character variables can be incremented but not decremented and
even so only plain ASCII letters and digits (a-z, A-Z and 0-9) are supported.
Incrementing/decrementing other character variables has no effect, the
original string is unchanged.
Example #1 Arithmetic Operations on Character Variables
<?php
echo '== Alphabetic strings ==' . PHP_EOL;
$s = 'W';
for ($n=0; $n<6; $n++) {
echo ++$s . PHP_EOL;
}
// Alphanumeric strings behave differently
echo '== Alphanumeric strings ==' . PHP_EOL;
$d = 'A8';
for ($n=0; $n<6; $n++) {
echo ++$d . PHP_EOL;
}
$d = 'A08';
for ($n=0; $n<6; $n++) {
echo ++$d . PHP_EOL;
}
?>
The above example will output:
== Alphabetic strings == X Y Z AA AB AC == Alphanumeric strings == A9 B0 B1 B2 B3 B4 A09 A10 A11 A12 A13 A14
Incrementing or decrementing booleans has no effect.