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
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 ?? null coalescing
left ? : ternary
right = += -= *= **= /= .= %= &= |= ^= <<= >>= ??= assignment
right yield from yield from
right yield yield
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\$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-1 . ", 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 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.

User Contributed Notes 11 notes

149
fabmlk
4 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. ```
21
aaronw at catalyst dot net dot nz
2 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 ```
42
Carsten Milkau
7 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. ```
ivan at dilber dot info
2 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);?> ```
karlisd at gmail dot com
4 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:<?phpfunction 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. ```
noone
1 month 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 5You'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  5We 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. ```
-2
wbrzezin
2 years ago
``` null coalescing `??` is between logic or `||`    and ternary `? :` ```
-6
headden at karelia dot ru
10 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 ```
-8
kitchin
3 years 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.?> ```
-17
leipie at gmail dot com
6 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! ```
-18
ohcc at 163 dot com
2 years ago
``` Syntax (new Person())->talk(); is supported as of PHP 5.5<?php    class A {        public \$b = 'B';        public function b(){            return 'Bee';        }    }    \$a = new A;    new \$a->b();// This means new B() rather than new Bee()?> ```