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
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 = $c).
Operators of equal precedence that are non-associative cannot be used
next to each other, for example 1 < 2 > 1 is
illegal in PHP. The expression 1 <= 1 == 1 on the
other hand is legal, because the == operator has a lower
precedence than the <= operator.
Associativity is only meaningful for binary (and ternary) operators.
Unary operators are either prefix or postfix so this notion is not applicable.
For example !!$a can only be grouped as !(!$a).
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.
| Associativity | Operators | Additional Information |
|---|---|---|
| (n/a) |
clone
new
|
clone and new |
| right | ** |
arithmetic |
| (n/a) |
+
-
++
--
~
(int)
(float)
(string)
(array)
(object)
(bool)
@
|
arithmetic (unary + and -),
increment/decrement,
bitwise,
type casting and
error control
|
| left | instanceof |
type |
| (n/a) | ! |
logical |
| left |
*
/
%
|
arithmetic |
| left |
+
-
.
|
arithmetic (binary + and -),
array and
string (. prior to PHP 8.0.0)
|
| left |
<<
>>
|
bitwise |
| left | . |
string (as of PHP 8.0.0) |
| non-associative |
<
<=
>
>=
|
comparison |
| non-associative |
==
!=
===
!==
<>
<=>
|
comparison |
| left | & |
bitwise and references |
| left | ^ |
bitwise |
| left | | |
bitwise |
| left | && |
logical |
| left | || |
logical |
| right | ?? |
null coalescing |
| non-associative | ? : |
ternary (left-associative prior to PHP 8.0.0) |
| right |
=
+=
-=
*=
**=
/=
.=
%=
&=
|=
^=
<<=
>>=
??=
|
assignment |
| (n/a) | yield from |
yield from |
| (n/a) | yield |
yield |
| (n/a) | print |
|
| left | and |
logical |
| left | xor |
logical |
| left | or |
logical |
Example #1 Associativity
<?php
$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
<?php
$a = 1;
echo $a + $a++; // may print either 2 or 3
$i = 1;
$array[$i] = $i++; // may set either index 1 or 2
?>
Example #3 +, - and . have the same precedence (prior to PHP 8.0.0)
<?php
$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
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 offoo()is put into $a.
Changelog
| Version | Description |
|---|---|
| 8.0.0 |
String concatenation (.) now has a lower precedence than
arithmetic addition/subtraction (+ and -) and
bitwise shift left/right (<< and >>);
previously it had the same precedence as + and -
and a higher precedence than << and >>.
|
| 8.0.0 |
The ternary operator (? :) is non-associative now;
previously it was left-associative.
|
| 7.4.0 |
Relying on the precedence of string concatenation (.) relative to
arithmetic addition/subtraction (+ or -) or
bitwise shift left/right (<< or >>),
i.e. using them together in an unparenthesized expression, is deprecated.
|
| 7.4.0 |
Relying on left-associativity of the ternary operator (? :),
i.e. nesting multiple unparenthesized ternary operators, is deprecated.
|
