PHP 8.1.0 Released!

运算符优先级

运算符优先级指定了两个表达式绑定得有多“紧密”。例如,表达式 1 + 5 * 3 的结果是 16 而不是 18 是因为乘号(“*”)的优先级比加号(“+”)高。必要时可以用括号来强制改变优先级。例如:(1 + 5) * 3 的值为 18

如果运算符优先级相同,那运算符的结合方向决定了该如何运算。例如,"-"是左联的,那么 1 - 2 - 3 就等同于 (1 - 2) - 3 并且结果是 -4. 另外一方面,"="是右联的,所以 $a = $b = $c 等同于 $a = ($b = $c)

没有结合的相同优先级的运算符不能连在一起使用,例如 1 < 2 > 1 在PHP是不合法的。但另外一方面表达式 1 <= 1 == 1 是合法的, 因为 == 的优先级低于 <=

关联性仅对二(三)元操作符有意义。 一元操作符是前缀或者后缀,所以不适用该概念。 例如 !!$a 仅可分为 !(!$a)

括号的使用,哪怕在不是必要的场合下,通过括号的配对来明确标明运算顺序,而非靠运算符优先级和结合性来决定,通常能够增加代码的可读性。

下表按照优先级从高到低列出了运算符。同一行中的运算符具有相同优先级,此时它们的结合方向决定求值顺序。

运算符优先级
结合方向 运算符 附加信息
不适用 clone new clonenew
** 算术运算符
不适用 + - ++ -- ~ (int) (float) (string) (array) (object) (bool) @ 算术 (一元 +-), 递增/递减按位类型转换错误控制
instanceof 类型
不适用 ! 逻辑运算符
* / % 算术运算符
+ - . 算数 (二元 +-), arraystring. PHP 8.0.0 前可用)
<< >> 位运算符
. string (PHP 8.0.0 起可用)
< <= > >= 比较运算符
== != === !== <> <=> 比较运算符
& 位运算符引用
^ 位运算符
| 位运算符
&& 逻辑运算符
|| 逻辑运算符
?? null 合并运算符
无关联 ? : 三元运算符 (PHP 8.0.0 之前左联)
= += -= *= **= /= .= %= &= |= ^= <<= >>= ??= 赋值运算符
不适用 yield from yield from
不适用 yield yield
不适用 print print
and 逻辑运算符
xor 逻辑运算符
or 逻辑运算符

示例 #1 结合方向

<?php
$a 
5// (3 * 3) % 5 = 4
// PHP 的三元操作符跟 C/C++ 有区别
$a true true 2// (true ? 0 : true) ? 1 : 2 = 2 (PHP 8.0.0 前可用)

$a 1;
$b 2;
$a $b += 3// $a = ($b += 3) -> $a = 5, $b = 5
?>

运算符优先级和关联方式仅决定表达式如何分组,不指定计算顺序。 一般情况下, PHP 不指定表达式的计算顺序,并且代码避免指定假设执行顺序, 因为行为会在 PHP 版本间发生变化或者依赖于旁边的代码。

示例 #2 未定义执行顺序

<?php
$a 
1;
echo 
$a $a++; // 可能会输出 2 或 3

$i 1;
$array[$i] = $i++; // 可能会设置索引 1 或 2
?>

示例 #3 +-. 具有相同的优先级

<?php
$x 
4;
// 这行可能会导致不可预料的输出:
echo "x minus one equals " $x-", or so I hope\n";
// 因为它是这样计算的:(PHP 8.0.0 之前版本)
echo (("x minus one equals " $x) - 1) . ", or so I hope\n";
// 可以使用括号来强制指定优先级:
echo "x minus one equals " . ($x-1) . ", or so I hope\n";
?>

以上例程会输出:

-1, or so I hope
-1, or so I hope
x minus one equals 3, or so I hope

注意:

尽管 = 比其它大多数的运算符的优先级低,PHP 仍旧允许类似如下的表达式:if (!$a = foo()),在此例中 foo() 的返回值被赋给了 $a

更新日志

版本 说明
8.0.0 现在,字符串连接符(.)的优先级比算数加/减(+-)、按位左/右移(<<>>)更低。在此之前,它的优先级与 +- 相同,并且比 <<>> 更高。
8.0.0 三元运算符(? :)是现在是非关联的;以前它是左联的。
7.4.0 已弃用在无括号的表达式中依赖字符串连接(.)相对于算数加/减(+ 或者 -)或者按位左/右移(<< 或者 >>)的优先级的使用方法。
7.4.0 不推荐使用三元运算符(? :)的左联。 即已弃用嵌套多个未带括号的三元运算符。
add a note add a note

User Contributed Notes 9 notes

up
178
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.
up
25
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
7
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);

?>
up
0
tlili dot mokhtar at gmail dot com
2 months 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
up
-1
karlisd at gmail dot com
6 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.
up
-2
instatiendaweb at gmail dot com
9 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.
up
-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.
up
-2
anisgazig at gmail dot com
10 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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