Operators
PHP Manual

Incrementing/Decrementing Operators

PHP supports C-style pre- and post-increment and decrement operators.

Note: The increment/decrement operators only affect numbers and strings. Arrays, objects and resources are not affected. Decrementing NULL values has no effect too, but incrementing them results in 1.

Increment/decrement Operators
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 alphabets 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 '== Alphabets ==' . PHP_EOL;
$s = 'W';
for ($n=0; $n<6; $n++) {
    echo ++$s . PHP_EOL;
}
// Digit characters behave differently
echo '== Digits ==' . 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:

== Characters ==
X
Y
Z
AA
AB
AC
== Digits ==
A9
B0
B1
B2
B3
B4
A09
A10
A11
A12
A13
A14

Incrementing or decrementing booleans has no effect.