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