PHP: Operator Precedence

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.

**Operator Precedence**
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`	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 of foo() 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.

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User Contributed Notes 8 notes

down

232

fabmlk ¶

9 years ago

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.

down

rvwoens at gmail dot com ¶

1 year ago

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')

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aaronw at catalyst dot net dot nz ¶

7 years ago

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: http://php.net/manual/en/tokens.php

down

Carsten Milkau ¶

12 years ago

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  == 1) do_something(); ?>

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

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

in PHP instead.

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sangala at seznam dot cz ¶

2 years ago

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)

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ivan at dilber dot info ¶

7 years ago

<?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);

?>

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tlili dot mokhtar at gmail dot com ¶

3 years ago

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

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instatiendaweb at gmail dot com ¶

3 years ago

//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.

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