Incrementing/Decrementing Operators

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

Note:

The increment/decrement operators do not affect boolean values. Decrementing NULL values has no effect too, but incrementing them results in 1.

Table 6.6. 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:

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<?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 Perl 'Z'+1 turns into 'AA', while in C 'Z'+1 turns into '[' ( ord('Z') == 90, ord('[') == 91 ). Note that character variables can be incremented but not decremented and even so only plain ASCII characters (a-z and A-Z) are supported.

Example 6.5. Arithmetic Operations on Character Variables

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<?php

$i = 'W';

for ($n=0; $n<6; $n++) {

    echo ++$i . "\n";

}
?>

The above example will output:

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X

Y

Z

AA

AB

AC

Incrementing or decrementing booleans has no effect.

Code Examples / Notes » language.operators.increment

Speed tip: Do not use post-incrementation/post-decrementation ($i++, $i--) where you do not work with the result of this expression. (For novices: Yes, every expression returns an result, also $a = '5' returns result, same as $a && $b. And this consumes more time and resources.) When writing loops, replace the post-incrementation with pre-incrementation, it is around 3times faster than post-incrementation. Why? In post-incrementation, PHP needs to copy variable value somewhere, then it increments the value, then returns the value which was stored before the incrementation was done. No matter if you don't expect the return value, PHP is scripting language, not compiled one, so it doesn't optimize use of return values. <?php // Good practice for loop: $array_count = count($array); // Store temporarily instead of calling everytime in loop for ($i = 0; $i < $max_count; ++$i) { // Use pre-incrementation here, it is faster // do something here } ?>

JMcCarthy AT CitiStreet DOT com: As for your March 31 post, at least in PHP version 4.3 this no longer holds for 'D'. Your point is still valid for 'e' or 'E' and worth noting. Your comment from May 12 is simply not true, although it might be a bug in your specific version of PHP but that would seem very strange. <?php $Align = array('a', 'b', 'c'); $i = 0; echo $Align[$i++]; // Prints 'a', as expected ?> It might be interesting to know that pre-/postincrement assumes a value of 0 for undefined variables, but pre-/postdecrement does not: <?php echo var_dump(++$foo); // int(1) echo var_dump(--$bar); // NULL! ?>

A more detailed explanation of the string incremant is: First of all it is checked wether the string is a standart representaion of a number wich is true if it equals the regex /^ *[+-]?[0-9]*(\.[0-9]|[0-9]\.)[0-9]*([eE]?[+-]?[0-9]+)?$/ but not the regex /\+\./ (no idea why). if it does, the type is changed to integer (if it equals /^ *[+-]?[0-9]+$/) or to float and then incremented by one. An empty string becomes the string "1". Otherwise if the last character is one of [0-8], [a-y] or [A-Y] it is incremented. If it is Z it puts it back to A, is z to a, if 9 to 0 and trys to do the same with the previouse character. If a character is reatched that is not in [0-9a-zA-Z], nothing is done anymore (that's why " Z" will increment to " A"). If the begining is reached a new caracter is prepended. "1" "a" or "A" depending on wether the first character was "9", "z" or "Z". If the last character was not [0-9a-zA-Z] the string isn't chaged. hope this helps someone

Incrementing/Decrementing Operators

Table 6.6. Increment/decrement Operators

Example 6.5. Arithmetic Operations on Character Variables

Code Examples / Notes » language.operators.increment

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