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
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.
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.