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 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.

Operator Precedence
Associativity Operators Additional Information
non-associative clone new clone and new
left [ array()
right ++ -- ~ (int) (float) (string) (array) (object) (bool) @ types and increment/decrement
non-associative instanceof types
right ! logical
left * / % arithmetic
left + - . arithmetic and string
left << >> bitwise
non-associative < <= > >= comparison
non-associative == != === !== <> comparison
left & bitwise and references
left ^ bitwise
left | bitwise
left && logical
left || logical
left ? : ternary
right = += -= *= /= .= %= &= |= ^= <<= >>= => assignment
left and logical
left xor logical
left or logical
left , many uses

Example #1 Associativity

<?php
$a 
5// (3 * 3) % 5 = 4
// ternary operator associativity differs from C/C++
$a true true 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

<?php
$a 
1;
echo 
$a $a++; // may print either 2 or 3

$i 1;
$array[$i] = $i++; // may set either index 1 or 2
?>

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 of foo() is put into $a.

Коментарии

Автор:
Beware the unusual order of bit-wise operators and comparison operators, this has often lead to bugs in my experience. For instance:

<?php if ( $flags MASK  == 1do_something(); ?>

will not do what you might expect from other languages. Use

<?php if (($flags MASK) == 1do_something(); ?>

in PHP instead.
2012-07-06 15:06:54
http://php5.kiev.ua/manual/ru/language.operators.precedence.html
Автор:
Watch out for the difference of priority between 'and vs &&' or '|| vs or':
<?php
$bool 
true && false;
var_dump($bool); // false, that's expected

$bool true and false;
var_dump($bool); // true, ouch!
?>
Because 'and/or' have lower priority than '=' but '||/&&' have higher.
2015-06-02 20:41:22
http://php5.kiev.ua/manual/ru/language.operators.precedence.html
<?php 
// Another tricky thing here is using && or || with ternary ?:
$x && $y $a $b// ($x && $y) ? $a : $b;

// while:
$x and $y $a $b// $x and ($y ? $a : $b);

?>
2017-04-24 18:55:49
http://php5.kiev.ua/manual/ru/language.operators.precedence.html
If you've come here looking for a full list of PHP operators, take note that the table here is *not* complete. There are some additional operators (or operator-ish punctuation tokens) that are not included here, such as "->", "::", and "...".

For a really comprehensive list, take a look at the "List of Parser Tokens" page: tokens
2017-08-11 04:18:29
http://php5.kiev.ua/manual/ru/language.operators.precedence.html
//incorrect
$a = true ? 0 : true ? 1 : 2; // (true ? 0 : true) ? 1 : 2 = 2
//Unparenthesized `a ? b : c ? d : e` is not supported. Use either `(a ? b : c) ? d : e` or `a ? b : (c ? d : e)` 
//correct
$a = (true ? 0 : true) ? 1 : 2; // (true ? 0 : true) ? 1 : 2 = 2

==> correction documentation.
2021-02-20 23:44:17
http://php5.kiev.ua/manual/ru/language.operators.precedence.html
An easy trick to get the result of the left shift operation (<<), e.g.

15 << 2 = 15 * (2*2) = 60

15 << 3 = 15 * (2*2*2) = 120

15 << 5 = 15 * (2*2*2*2*2) = 480

and so on...

So it's:

(number on left) multiplied by (number on right) times 2.

The same goes for the right shift operator (>>), where:

(number on left) divided by (number on right) times 2 e.g.

15 >> 2 = (15/2)/2 = 7/2 = 3 (use floor values if result is in decimals).

35 >> 3 = (((35/2)/2)/2 = (17/2)/2 = 8/2 = 4
2021-09-05 00:58:58
http://php5.kiev.ua/manual/ru/language.operators.precedence.html
Using cast and ternary operator can be unclear,
(Useful to know with: declare(strict_types = 1) ). 
<?php
$num_str
="5";

$i1 = (int)  isset($num_str) ? $num_str 0;
$i2 = (int) (isset($num_str) ? $num_str 0);
var_dump($i1);
var_dump($i2);
?>
Output:
string(1) "5"
int(5)
2022-11-01 22:42:03
http://php5.kiev.ua/manual/ru/language.operators.precedence.html
Note that ?? has a low priority, so this can lead to unexpected results:

$a=[];
$a['aa']??'not set' 
--> not set (as expected)

but
"lets see if it is set".$a['aa']??'not set'
--> notice; undefined index aa
--> lets see if it is set

so you need to use parenthesis
"lets see if it is set".($a['aa']??'not set')
2023-01-31 02:11:58
http://php5.kiev.ua/manual/ru/language.operators.precedence.html

    Поддержать сайт на родительском проекте КГБ