Incrementing/Decrementing Operators

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

Table 15-5. 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 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.
Example 15-3. Arithmetic Operations on Character Variables

<?php $i = 'W'; for($n=0; $n<6; $n++) echo ++$i . "\n";
The above example will output:
X Y Z AA AB AC

Incrementing or decrementing booleans has no effect.

User Contributed Notes
Incrementing/Decrementing Operators

timo at frenay dot net
25-Aug-2004 10:45


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!

?>

JMcCarthy AT CitiStreet DOT com
12-May-2004 11:39


Be aware that $Align[$i++] will produce an error even if $Align is an actual array and the indexes are numeric and the value of $i begins at zero.

31-Mar-2004 03:19


Note that incrementing strings can give unpredictable results due to type changes.  For example:



<?php



$i = '9C6';

for($n=0; $n<10; $n++)

  echo ++$i . "\n";



?>



Gives you:

 9C7

 9C8

 9C9

 9D0

 10

 11

 12

..etc.



The 'D' (and also 'E') characters are interpreted here as exponents of 10 (i.e., scientific notation) formatted numbers.  Using '9D6' will give 9000001, 9000002, etc.



You might want to use all alphabetical or all numerical, but not mix the two otherwise you may not get what you expect..

chris at free-source dot com
06-Feb-2004 06:11


Interesting performance note:





$i++ seems to be slightly slower than ++$i, when used on a line by itself the 2 have the same purpose.  It's not much, but over 100,000 incements the pre-increment is about .004 seconds faster on average.

mu at despammed dot net
14-Oct-2002 11:11


The exact moment when post-increment and post-decrement happen is _just immediately after the variable is evaluated_ (not "after the line is processed" or something like that)



Example 1:

$i = 2;

echo $i++ + $i;

Result: 5. The first i is evaluated as 2, gets incremented to 3. i is then evaluated as 3 for the second occurance.



Example 2:

$i = 2;

echo $i + $i++;

Result: 4. The first i is 2. Second i is 2 too, gets incremented afterwards.

l dot bergmann at donz dot de
24-Sep-2002 02:50


Be carefull with expressions like

$a += ++$a + $a++;

Had a lot of fun with it: Different languages calculated different results.

Using $a = 12 you might get something between 38 and 40...

cleong at letstalk dot com
17-Oct-2001 09:52


Note that the ++ and -- don't convert a boolean to an int. The following code will loop forever.





function a($start_index) {


for($i = $start_index; $i < 10; $i++) echo "\$i = $i\n";


}





a(false);





This behavior is, of course, very different from that in C. Had me pulling out my hair for a while.

fred at surleau dot com
18-Jul-2001 02:02


Other samples :


$l="A";      $l++; -> $l="B"


$l="A0";     $l++; -> $l="A1"


$l="A9";     $l++; -> $l="B0"


$l="Z99";    $l++; -> $l="AA00"


$l="5Z9";    $l++; -> $l="6A0"


$l="9Z9";    $l++; -> $l="10A0"


$l="9z9";    $l++; -> $l="10a0"


$l="J85410"; $l++; -> $l="J85411"


$l="J99999"; $l++; -> $l="K00000"


$l="K00000"; $l++; -> $l="K00001"