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.
|non-associative||clone new||clone and new|
|right||~ - (int) (float) (string) (array) (object) (bool) @||types|
|left||* / %||arithmetic|
|left||+ - .||arithmetic and string|
|non-associative||< <= > >= <>||comparison|
|non-associative||== != === !==||comparison|
|left||&||bitwise and references|
|right||= += -= *= /= .= %= &= |= ^= <<= >>= =>||assignment|
For operators of equal precedence, left associativity means that evaluation proceeds from left to right, and right associativity means the opposite.
Example #1 Associativity
$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 + $a++; // may print 4 or 5
Although = has a lower precedence than most other operators, PHP will still allow expressions similar to the following: if (!$a = foo()), in which case the return value of foo() is put into $a.