The precedence of an operator specifies how "tightly" it binds two
expressions together. For example, in the expression
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
(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
$a = $b = $c is grouped as
$a = ($b = $c).
Operators of equal precedence that are non-associative cannot be used
next to each other, for example
1 < 2 > 1 is
illegal in PHP. The expression
1 <= 1 == 1 on the
other hand is legal, because the
== operator has a lower
precedence than the
Associativity is only meaningful for binary (and ternary) operators.
Unary operators are either prefix or postfix so this notion is not applicable.
!!$a can only be grouped as
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.
||clone and new|
||string (as of PHP 8.0.0)|
||bitwise and references|
||ternary (left-associative prior to PHP 8.0.0)|
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 (prior to PHP 8.0.0)
$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
. have the same precedence (prior to PHP 8.0.0)
$x = 4;
// this line might result in unexpected output:
echo "x minus one equals " . $x-1 . ", or so I hope\n";
// because it is evaluated like this line (prior to PHP 8.0.0):
echo (("x minus one equals " . $x) - 1) . ", or so I hope\n";
// the desired precedence can be enforced by using parentheses:
echo "x minus one equals " . ($x-1) . ", or so I hope\n";
The above example will output:
-1, or so I hope -1, or so I hope x minus one equals 3, or so I hope
=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.
String concatenation (
The ternary operator (
Relying on the precedence of string concatenation (
Relying on left-associativity of the ternary operator (