Operator Precedence

## Operator Precedence

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.

Operator Precedence
non-associative clone new clone and new
left [ array()
right ++ -- ~ (int) (float) (string) (array) (object) (bool) @ types and increment/decrement
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. For operators of equal precedence that are non-associative those operators may not associate with themselves. So for example, the statement 1 < 2 > 1, is illegal in PHP. Whereas, the statement 1 <= 1 == 1 is not, because the `T_IS_EQUAL` operator has lesser precedence than the `T_IS_SMALLER_OR_EQUAL` operator.

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?> ```
Use of parentheses, even when not strictly necessary, can often increase readability of the code.

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.