## Arithmetic Operators

Remember basic arithmetic from school? These work just like those.

Arithmetic Operators
Example Name Result
-\$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 Modulus Remainder of \$a divided by \$b.

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.

Operands of modulus are converted to integers (by stripping the decimal part) before processing.

The result of the modulus operator % has the same sign as the dividend — that is, the result of \$a % \$b will have the same sign as \$a. For example:

``` <?phpecho (5 % 3)."\n";           // prints 2echo (5 % -3)."\n";          // prints 2echo (-5 % 3)."\n";          // prints -2echo (-5 % -3)."\n";         // prints -2?> ```

### User Contributed Notes 13 notes

54
Jonathon Reinhart
11 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>";}   }?> ```
pr dot dot dot dot k at g dot dot dot com
1 year ago
``` 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 ```
info at sima-pc dot com
14 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. ```
-7
arjini at gmail dot com
13 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. ```
-4
philippe
6 months ago
``` To get a positiv resultfunction 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);} ```
-10
TheWanderer
9 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). ```
-10
Dominik Buechler
3 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. ```
-14
lmc at trendicy dot com
3 years ago
``` If you are running a php version older than 5.6, you can calculate \$a ** \$b by using exp(\$b*log(\$a)) ```
-20
Andrew
4 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. ```
-9
peter at icb dot at
10 months ago
``` If you need the mathematical modulo (always positive) from negative numbers, use this small function:<?phpfunction modulo(\$a , \$b) { return (\$a + \$b) % \$b; }// examples:echo modulo(15, 12);  // 3echo modulo(-9, 12);  // 3?> ```
-27
calmarius at atw dot hu
10 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) ```
-30
glenn at benge dot co dot nz
13 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. ```
-45
php at richardneill dot org
7 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) ```