Remember basic arithmetic from school? These work just like those.
|+$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:
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.