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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 
5// (3 * 3) % 5 = 4
// ternary operator associativity differs from C/C++
$a true true 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-", 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.
add a note add a note

User Contributed Notes 9 notes

up
174
fabmlk
6 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.
up
49
Carsten Milkau
9 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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24
aaronw at catalyst dot net dot nz
4 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
up
9
ivan at dilber dot info
4 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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-1
tlili dot mokhtar at gmail dot com
1 month 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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-1
karlisd at gmail dot com
5 years ago
Sometimes it's easier to understand things in your own examples.
If you want to play around operator precedence and look which tests will be made, you can play around with this:

<?php
function F($v) {echo $v." "; return false;}
function
T($v) {echo $v." "; return true;}

IF (
F(0) || T(1) && F(2)  || F(3)  && ! F(4) ) {
  echo
"true";
} else echo
" false";
?>
Now put in IF arguments f for false and t for true, put in them some ID's. Play out by changing "F" to "T" and vice versa, by keeping your ID the same. See output and you will know which arguments  actualy were checked.
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-2
instatiendaweb at gmail dot com
8 months 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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-2
noone
1 year ago
Something that threw me of guard and I hadn't found it mentioned anywhere is if you're looking to asign a value in an if statement condition and use the same value in the said condition and compare it to a different value note the precedence of operators.

if($a=5&&$a==5){
  echo '5';
} else {
  echo 'not 5';
}
//echos  not 5

You'll get a Notice:  Undefined variable: a;
This happens because the expression is treated as
($a=5&&($a==5))
In this case $a was undefined.

Use parentheses to enforce the desired outcome or and instead of &&.
if(($a=5)&&$a==5){ // or $a=5 and $a==5
  echo '5';
} else {
  echo 'not 5';
}

//echos  5

We get no notice!

A use case for this can be a three part condition that first checks if a value is valid, second assigns a new variable based on the first value and then checks if the result is valid.

$ID=100;

if ($ID&&($data=get_table_row_for_ID($ID))&&$data->is_valid()) { //NOTE: assigned $data
// do something with the data
}

If assigning variables in an if condition I recommend adding a comment at the end of the line that such an action took place.
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-2
anisgazig at gmail dot com
8 months ago
Three types of operator associativity in php.
1.left
2.rigt
3.non-associativity

Category of three operators are right associativity
1)**
2)=,+=,-=,*=,/=,%=,&=,^=,|=,<<=,>>=,??=,.=
3)??

Category of eight operators are non-associativity
1)clone new
2)++,--,~,@
3)!
4)<,<=,>,>=
5)<<,>>
6)yield from
7)yield
8)print

Rest of the operators are left associativity
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