ScotlandPHP

## 运算符优先级

clone new clonenew
[ array()
** 算术运算符
++ -- ~ (int) (float) (string) (array) (object) (bool) @ 类型递增／递减
instanceof 类型
! 逻辑运算符
* / % 算术运算符
+ - . 算术运算符字符串运算符
<< >> 位运算符
< <= > >= 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
?? 比较运算符
? : ternary
right = += -= *= **= /= .= %= &= |= ^= <<= >>= 赋值运算符
and 逻辑运算符
xor 逻辑运算符
or 逻辑运算符

Example #1 结合方向

``` <?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:

### User Contributed Notes 11 notes

81
fabmlk
2 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. ```
41
Carsten Milkau
5 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. ```
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:<?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. ```
-1
muhehahe
14 days ago
``` \$item = array_pop(\$this->stack) && !\$this->stop) // \$item = 1(\$item = array_pop(\$this->stack)) && !\$this->stop) // \$item = array_pop(\$this->stack)Both in while loop pop whole stack or work until stopped, but first one is somehow errorous. ```
-1
kitchin
10 months 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.?> ```
-3
8 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 ```
-1
ivan at dilber dot info
5 months 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);?> ```
-1
aaronw at catalyst dot net dot nz
2 months 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 ```
-8
leipie at gmail dot com
4 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! ```
-22
Anonymous
2 years 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';} ```
-3
ohcc at 163 dot com
2 months 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()?> ```