The modulus operator is very poorly suited for such a simple operation as determining if an int is even or odd. On most common systems, modulus performs a division, which is a very slow operation.
A much better way to find if a number is even or odd is to use the bitwise & operator.
e.g.
$is_odd = $x & 1; //using and
$is_odd = $x % 2; //using modulus
Arithmetic Operators
Remember basic arithmetic from school? These work just like those.
| Example | Name | Result |
|---|---|---|
| +$a | Identity | Conversion of $a to int or float as appropriate. |
| -$a | Negation | Opposite of $a. |
| $a + $b | Addition | Sum of $a and $b. |
| $a - $b | Subtraction | Difference of $a and $b. |
| $a * $b | Multiplication | Product of $a and $b. |
| $a / $b | Division | Quotient of $a and $b. |
| $a % $b | Modulo | Remainder of $a divided by $b. |
| $a ** $b | Exponentiation | Result of raising $a to the $b'th power. Introduced in PHP 5.6. |
The division operator ("/") returns a float value unless the two operands are integers (or strings that get converted to integers) and the numbers are evenly divisible, in which case an integer value will be returned. For integer division, see intdiv().
Operands of modulo are converted to integers (by stripping the decimal part) before processing. For floating-point modulo, see fmod().
The result of the modulo operator % has the same sign as the dividend — that is, the result of $a % $b will have the same sign as $a. For example:
<?php
echo (5 % 3)."\n"; // prints 2
echo (5 % -3)."\n"; // prints 2
echo (-5 % 3)."\n"; // prints -2
echo (-5 % -3)."\n"; // prints -2
?>
See also the manual page on Math functions.
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User Contributed Notes 14 notes
pr dot dot dot dot k at g dot dot dot com ¶
2 years ago
Jonathon Reinhart ¶
12 years ago
A very simple yet maybe not obvious use of the modulus (%) operator is to check if an integer is odd or even.
<?php
if (($a % 2) == 1)
{ echo "$a is odd." ;}
if (($a % 2) == 0)
{ echo "$a is even." ;}
?>
This is nice when you want to make alternating-color rows on a table, or divs.
<?php
for ($i = 1; $i <= 10; $i++) {
if(($i % 2) == 1) //odd
{echo "<div class=\"dark\">$i</div>";}
else //even
{echo "<div class=\"light\">$i</div>";}
}
?>
biziclop at vipmail dot hu ¶
3 months ago
For positive modulos, here is a mathematically proper modulo which actually works with negative integers.
<?php
// Inline: $v is value to be divided, $m is the modulus
$remainder = ( $v % $m + $m ) % $m;
// Or as a function:
function modulo( $value, $modulus ){
return ( $value % $modulus + $modulus ) % $modulus;
}
// Test:
for( $x = -6; $x <= 6; ++$x ) echo $x, "\t", modulo( $x, 3 ), "\t<br>\n";
/* prints:
-6 0
-5 1
-4 2
-3 0
-2 1
-1 2
0 0
1 1
2 2
3 0
*/ ?>
info at sima-pc dot com ¶
15 years ago
Note that operator % (modulus) works just with integers (between -214748348 and 2147483647) while fmod() works with short and large numbers.
Modulus with non integer numbers will give unpredictable results.
philippe ¶
1 year ago
To get a positiv result
function modulo(int $a, int $b):?int {
if ($b == 0) {
throw new Exception('modulo : second operand must not be zero');
}
$b = abs($b);
// test $b == 1 for performance when $a < 0
return ($b == 1) ? 0 : (($a < 0) ? modulo($a + $b, $b) : $a % $b);
}
arjini at gmail dot com ¶
15 years ago
When dealing purely with HTML, especially tables, or other things in "grids" the modulous operator is really useful for splitting up the data with a seperator.
This snippet reads any gif files from the directory the script is in, prints them out and puts in a break every 5th image.
<?php
$d = dir('./');
$i = 0;
while(false !== ($e = $d->read())){
if(strpos($e,'.gif')){
++$i;
echo '<img src="'.$e.'"/>'.chr(10);
if(!($i%5))
echo '<br/>';
}
}
?>
For tables just put </tr><tr> in place of the break.
TheWanderer ¶
11 years ago
It is worth noticing that when working with large numbers, most noticably using the modulo operator, the results depend on your CPU architecture. Therefore, running a decent 64-bit machine will be to your advantage in case you have to perform complex mathematical operations. Here is some example code - you can compare its output on x86 and x86_64 machines:
<?php
/* tested under PHP 5.2.6-1 with Suhosin-Patch 0.9.6.2 (cli) on both i386 and amd64, Debian lenny/sid */
$a = 2863311530;
$b = 256;
$c = $a % $b;
echo "$c <br />\n";
echo (2863311530 % 256)." <br />\n"; /* directly with no variables, just to be sure */
?>
The code is expected to produce '170' if working correctly (try it in spreadsheet software).
Dominik Buechler ¶
4 years ago
In addition to Jonathan's comment, there is a way simpler way to determine if an integer is even or not:
<? $odd = $i % 2; ?>
or
<? $even = !($i % 2); ?>
This works because a modulo division by 2 will always return either 0 or the rest 1. Since those are valid boolean values you can just invert them by adding a prefixed ! if wanted.
Andrew ¶
6 years ago
The % operator doesn't behave as many people with a maths background would expect, when dealing with negative numbers. For example, -1 mod 8 = 7, but in PHP, -1 % 8 = -1.
The following function has the expected behaviour:
function mod($a, $n) {
return ($a % $n) + ($a < 0 ? $n : 0);
}
mod(-1, 8) returns 7 as expected.
lmc at trendicy dot com ¶
5 years ago
If you are running a php version older than 5.6, you can calculate $a ** $b by using exp($b*log($a))
peter at icb dot at ¶
2 years ago
If you need the mathematical modulo (always positive) from negative numbers, use this small function:
<?php
function modulo($a , $b) { return ($a + $b) % $b; }
// examples:
echo modulo(15, 12); // 3
echo modulo(-9, 12); // 3
?>
calmarius at atw dot hu ¶
11 years ago
Be careful when using % with large numbers.
The code:
<?php
echo 3333333333 % 3
?>
puts out -1 instead of zero!
(Due to the overflow)
glenn at benge dot co dot nz ¶
14 years ago
a real simple method to reset an integer to a the next lowest multiple of a divisor
$startSeq = $startSeq - ($startSeq % $entriesPerPage);
if $startSeq was already a multiple, then " $startSeq % $entriesPerPage " will return 0 and $startSeq will not change.
php at richardneill dot org ¶
8 years ago
For larger numbers (above PHP_INT_MAX), use fmod() rather than %.
The other operators (+-*/) work correctly with floats and integer overflow, but % uses integer wrap. Eg.
<?php
var_dump(0xffffffff % 2);
//Prints int(-1) which is WRONG
var_dump(intval(fmod(0xffffffff,2)));
//Prints int(1) which is the right answer
?>
(The reason this matters is that PHP's float is actually a double, and can accurately represent integers up to 52-bits, even on 32-bit systems)
