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.
| 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 = 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] = $i++; // may set either index 1 or 2
?>
Note:
Although = has a lower precedence than most other operators, PHP will still allow expressions similar to the following: if (!$a = foo()), in which case the return value of foo() is put into $a.
BEWARE: Addition, subtraction, and string concatenation have equal precedence!
<?
$x = 4;
echo "x minus one equals " . $x-1 . ", or so I hope";
?>
will print "-1, or so I hope"
(Concatenate the first string literal and the value of $x, then implicitly convert that to a number (zero) so you can subtract 1 from it, then concatenate the final string literal.)
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.
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.
Too add something ton Antistone's comment :
{Antistone ΒΆ
1 year ago
BEWARE: Addition, subtraction, and string concatenation have equal precedence!
<?
$x = 4;
echo "x minus one equals " . $x-1 . ", or so I hope";
?>
will print "-1, or so I hope"
(Concatenate the first string literal and the value of $x, then implicitly convert that to a number (zero) so you can subtract 1 from it, then concatenate the final string literal.)
}
You can use the operator "," instead of ".".
This way allows you to concatenate the first string literal after $x-1.
So the first string literal is note convert to a number (zero) .
Solution :
<?php
$x = 4;
echo "x minus one equals " , $x-1 . ", or so I hope";
?>
will print :
"x minus one equals 3, or so I hope "
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.
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
Another way to sort out the problem mentioned by "nicoolasens at gmail dot com" and "Antistone" is:
<?php
$x = 4;
echo "x minus one equals " . $x-1 . ", or so I hope";
?>
will print "-1, or so I hope"
Solution:
We can wrap the methematical part in "()" and let the parser tell to solve it first in other words here we can get the benefit of higher precedence of "()" So if we rewrite the example as:
<?php
$x = 4;
echo "x minus one equals " . ($x-1) . ", or so I hope";
?>
The answer will be according to our expectation which is:
"x minus one equals 3, or so I hope"
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!
The following example will output false
$a = 1;
$b = 1;
$c = $a + $a++;
$d = 1 + $b++;
if($c == $d){
echo 'true';
}else{
echo 'false';
}
