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.
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.
| Associativity | Operators | Additional Information |
|---|---|---|
| (n/a) |
clone
new
|
clone and new |
| right | ** |
arithmetic |
| (n/a) |
++
--
~
(int)
(float)
(string)
(array)
(object)
(bool)
@
|
types and increment/decrement |
| left | instanceof |
types |
| (n/a) | ! |
logical |
| left |
*
/
%
|
arithmetic |
| left |
+
-
.
|
arithmetic și string |
| left |
<<
>>
|
bitwise |
| non-associative |
<
<=
>
>=
|
comparison |
| non-associative |
==
!=
===
!==
<>
<=>
|
comparison |
| left | & |
bitwise și references |
| left | ^ |
bitwise |
| left | | |
bitwise |
| left | && |
logical |
| left | || |
logical |
| right | ?? |
null coalescing |
| left | ? : |
ternary |
| right |
=
+=
-=
*=
**=
/=
.=
%=
&=
|=
^=
<<=
>>=
??=
|
assignment |
| (n/a) | yield from |
yield from |
| (n/a) | yield |
yield |
| (n/a) | 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
$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";
?>
Exemplul de mai sus va afișa:
-1, or so I hope -1, or so I hope x minus one equals 3, or so I hope
Notă:
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 offoo()is put into $a.
add a note
User Contributed Notes 8 notes
fabmlk ¶
9 years ago
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
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)
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.
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);
?>
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
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')
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.
