The precedence of an operator specifies how "tightly" it binds two expressions together. For example, in the expression 1 + 5 * 3, the answer is 16 and not 18 because the multiplication ("*") operator has a higher precedence than the addition ("+") operator. Parentheses may be used to force precedence, if necessary. For instance: (1 + 5) * 3 evaluates to 18.
When operators have equal precedence, their associativity decides whether they are evaluated starting from the right, or starting from the left - see the examples below.
The following table lists the operators in order of precedence, with the highest-precedence ones at the top. Operators on the same line have equal precedence, in which case associativity decides the order of evaluation.
| Associativity | Operators | Additional Information |
|---|---|---|
| non-associative | clone new | clone and new |
| left | [ | array() |
| non-associative | ++ -- | increment/decrement |
| right | ~ - (int) (float) (string) (array) (object) (bool) @ | types |
| non-associative | instanceof | types |
| right | ! | logical |
| left | * / % | arithmetic |
| left | + - . | arithmetic and string |
| left | << >> | bitwise |
| non-associative | < <= > >= <> | comparison |
| non-associative | == != === !== | comparison |
| left | & | bitwise and references |
| left | ^ | bitwise |
| left | | | bitwise |
| left | && | logical |
| left | || | logical |
| left | ? : | ternary |
| right | = += -= *= /= .= %= &= |= ^= <<= >>= => | assignment |
| left | and | logical |
| left | xor | logical |
| left | or | logical |
| left | , | many uses |
For operators of equal precedence, left associativity means that evaluation proceeds from left to right, and right associativity means the opposite.
Example #1 Associativity
<?php
$a = 3 * 3 % 5; // (3 * 3) % 5 = 4
$a = true ? 0 : true ? 1 : 2; // (true ? 0 : true) ? 1 : 2 = 2
$a = 1;
$b = 2;
$a = $b += 3; // $a = ($b += 3) -> $a = 5, $b = 5
// mixing ++ and + produces undefined behavior
$a = 1;
echo ++$a + $a++; // may print 4 or 5
?>
Note:
Although = has a lower precedence than most other operators, PHP will still allow expressions similar to the following: if (!$a = foo()), in which case the return value of foo() is put into $a.