PHP Manual: Incrementing/Decrementing Operators

Incrementing/Decrementing Operators

PHP supports pre- and post-increment and decrement operators. Those unary operators allow to increment or decrement the value by one.

**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 'Post-increment:', PHP_EOL;
$a = 5;
var_dump($a++);
var_dump($a);

echo 'Pre-increment:', PHP_EOL;
$a = 5;
var_dump(++$a);
var_dump($a);

echo 'Post-decrement:', PHP_EOL;
$a = 5;
var_dump($a--);
var_dump($a);

echo 'Pre-decrement:', PHP_EOL;
$a = 5;
var_dump(--$a);
var_dump($a);
?>

The above example will output:

Post-increment:
int(5)
int(6)
Pre-increment:
int(6)
int(6)
Post-decrement:
int(5)
int(4)
Pre-decrement:
int(4)
int(4)

Warning

The increment and decrement operators have no effect on values of type bool. A E_WARNING is emitted as of PHP 8.3.0, because this will implicitly cast the value to int in the future.

The decrement operator has no effect on values of type null. A E_WARNING is emitted as of PHP 8.3.0, because this will implicitly cast the value to int in the future.

The decrement operator has no effect on non- numeric string. A E_WARNING is emitted as of PHP 8.3.0, because a TypeError will be thrown in the future.

Note:
Internal objects that support overloading addition and/or subtraction can also be incremented and/or decremented. One such internal object is GMP.

PERL string increment feature

Warning

This feature is soft-deprecated as of PHP 8.3.0. The str_increment function should be used instead.

It is possible to increment a non- numeric string in PHP. The string must be an alphanumerical ASCII string. Which increments letters up to the next letter, when reaching the letter Z the increment is carried to the value on the left. For example, $a = 'Z'; $a++; turns $a into 'AA'.

Example #1 PERL string increment example

<?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

Warning

If the alphanumerical string can be interpreted as a numeric string it will be cast to an int or float. This is particularly an issue with strings that look like a floating point numbers written in exponential notation. The str_increment function does not suffer from these implicit type cast.

Example #2 Alphanumerical string converted to float

<?php
$s = "5d9";
var_dump(++$s);
var_dump(++$s);
?>

The above example will output:

string(3) "5e0"
float(6)

This is because the value "5e0" is interpreted as a float and cast to the value 5.0 before being incremented.