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
    ExampleNameEffect
    ++$aPre-incrementIncrements $a by one, then returns $a.
    $a++Post-incrementReturns $a, then increments $a by one.
    --$aPre-decrementDecrements $a by one, then returns $a.
    $a--Post-decrementReturns $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.

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