Operatori aritmetici

Ricordate l'aritmetica di base dalla scuola? Questi operatori funzionano esattamente nello stesso modo.

Operatori aritmetici
Esempio Nome Risultato
-$a Negazione Opposto di $a.
$a + $b Addizione La somma di $a e $b.
$a - $b Sottrazione La differenza di $a e $b.
$a * $b Moltiplicazione il prodotto di $a e $b.
$a / $b Divisione Quoziente di $a e $b.
$a % $b Modulo Il resto di $a diviso da $b.

L'operatore di divisione ("/") restituisce un valore float a meno che i due operandi siano interi (oppure stringhe che vengono convertite in interi) e i numeri siano divisibili, nel qual caso viene restituito un valore intero.

Gli operandi del modulo sono convertiti a interi (rimuovendo la parte decimale) prima dell'operazione.

Il risultato dell'operatore modulo % ha lo stesso segno del dividendo - ovvero, il risultato di $a % $b avrà lo stesso segno di $a. Per esempio:


echo (3)."\n";           // prints 2
echo (% -3)."\n";          // prints 2
echo (-3)."\n";          // prints -2
echo (-% -3)."\n";         // prints -2


Vedere anche le pagine del manuale sulle funzioni matematiche.

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User Contributed Notes 11 notes

Jonathon Reinhart
10 years ago
A very simple yet maybe not obvious use of the modulus (%) operator is to check if an integer is odd or even.
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.

for ($i = 1; $i <= 10; $i++) {
$i % 2) == 1//odd
{echo "<div class=\"dark\">$i</div>";}
{echo "<div class=\"light\">$i</div>";}
pr dot dot dot dot k at g dot dot dot com
1 month 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.


$is_odd = $x & 1; //using and
$is_odd = $x % 2; //using modulus
info at sima-pc dot com
12 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.
arjini at gmail dot com
12 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.

= dir('./');
$i = 0;
false !== ($e = $d->read())){
'<img src="'.$e.'"/>'.chr(10);

For tables just put </tr><tr> in place of the break.
lmc at trendicy dot com
2 years ago
If you are running a php version older than 5.6, you can calculate $a ** $b by using exp($b*log($a))
Dominik Buechler
2 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; ?>
<? $even = !($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.
8 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:
/* tested under PHP 5.2.6-1 with Suhosin-Patch (cli) on both i386 and amd64, Debian lenny/sid */
$a = 2863311530;
$b = 256;
$c = $a % $b;
"$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).
3 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.
calmarius at atw dot hu
8 years ago
Be careful when using % with large numbers.

The code:

echo 3333333333 % 3

puts out -1 instead of zero!

(Due to the overflow)
glenn at benge dot co dot nz
12 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
5 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.

(0xffffffff % 2);
//Prints  int(-1)   which is WRONG

//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)
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