PHP 7.1.0 Released

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 ** arithmetic
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
right ?? comparison
left ? : ternary
right = += -= *= **= /= .= %= &= |= ^= <<= >>= assignment
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

$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

<?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:
echo (("x minus one equals " $x) - 1) . ", or so I hope\n";
// the desired precendence can be enforced by using parentheses:
echo "x minus one equals " . ($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.

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User Contributed Notes 7 notes

up
50
fabmlk
1 year 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.
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31
Carsten Milkau
4 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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2
karlisd at gmail dot com
1 year 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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4
headden at karelia dot ru
7 years ago
Although example above already shows it, I'd like to explicitly state that ?: associativity DIFFERS from that of C++. I.e. convenient switch/case-like expressions of the form

$i==1 ? "one" :
$i==2 ? "two" :
$i==3 ? "three" :
"error";

will not work in PHP as expected
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-1
kitchin
15 days ago
The precedence of '->' is less than '[' in this situation: object contains array, name of array is stored in string variable.
<?php
$farm
= new StdClass;
$farm->emu = array( 'name' => 'Henry', 'age' => 9 );
$farm->rabbit = array( 'name' => 'George', 'age' => 4 );

$animal = 'rabbit';
print_r( $farm->$animal ); // ok
// print( $farm->$animal[ 'name' ] );  // wrong, [ has precedence.
print( $farm->{$animal}[ 'name' ] ); // correct, prints George.

$farm->wash = 'Suds';
$jobs = array( 'morning' => 'feed', 'evening' => 'wash' );
print(
$farm->$jobs[ 'evening' ] ); // correct, prints Suds.
print( $farm->{$jobs[ 'evening' ]} ); // correct, prints Suds.
?>
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-4
leipie at gmail dot com
3 years ago
The precedence of the arrow operator (->) on objects seems to the highest of all, even higher then clone.

But you can't wrap (clone $foo)->bar() like this!
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-8
Anonymous
1 year ago
The following example will output false
$a = 1;
$b = 1;

$c = $a + $a++;
$d = 1 + $b++;

if($c == $d){
    echo 'true';
}else{
    echo 'false';
}
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