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 how the operators are grouped. For example "-" is left-associative, so 1 - 2 - 3 is grouped as (1 - 2) - 3 and evaluates to -4. "=" on the other hand is right-associative, so $a = $b = $c is grouped as $a = ($b = $c).
Operators of equal precedence that are non-associative cannot be used next to each other, for example 1 < 2 > 1 is illegal in PHP. The expression 1 <= 1 == 1 on the other hand is legal, because the == operator has lesser precedence than the <= operator.
Use of parentheses, even when not strictly necessary, can often increase readability of the code by making grouping explicit rather than relying on the implicit operator precedence and associativity.
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 grouping.
|non-associative||clone new||clone and new|
|right||++ -- ~ (int) (float) (string) (array) (object) (bool) @||types and increment/decrement|
|left||* / %||arithmetic|
|left||+ - .||arithmetic and string|
|non-associative||< <= > >=||comparison|
|non-associative||== != === !== <>||comparison|
|left||&||bitwise and references|
|right||= += -= *= **= /= .= %= &= |= ^= <<= >>= =>||assignment|
Example #1 Associativity
$a = 3 * 3 % 5; // (3 * 3) % 5 = 4
// ternary operator associativity differs from C/C++
$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
Operator precedence and associativity only determine how expressions are grouped, they do not specify an order of evaluation. PHP does not (in the general case) specify in which order an expression is evaluated and code that assumes a specific order of evaluation should be avoided, because the behavior can change between versions of PHP or depending on the surrounding code.
Example #2 Undefined order of evaluation
$a = 1;
echo $a + $a++; // may print either 2 or 3
$i = 1;
$array[$i] = $i++; // may set either index 1 or 2
Although = has a lower precedence than most other operators, PHP will still allow expressions similar to the following: if (!$a = foo()), in which case the return value of foo() is put into $a.