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 <=。
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。
| 结合方向 | 运算符 | 附加信息 |
|---|---|---|
| 无 | clone new | clone 和 new |
| 左 | [ | array() |
| 右 | ** | 算术运算符 |
| 右 | ++ -- ~ (int) (float) (string) (array) (object) (bool) @ | 类型和递增／递减 |
| 无 | instanceof | 类型 |
| 右 | ! | 逻辑运算符 |
| 左 | * / % | 算术运算符 |
| 左 | + - . | 算术运算符和字符串运算符 |
| 左 | << >> | 位运算符 |
| 无 | < <= > >= | 比较运算符 |
| 无 | == != === !== <> <=> | 比较运算符 |
| 左 | & | 位运算符和引用 |
| 左 | ^ | 位运算符 |
| 左 | | | 位运算符 |
| 左 | && | 逻辑运算符 |
| 左 | || | 逻辑运算符 |
| 左 | ?? | 比较运算符 |
| 左 | ? : | ternary |
| right | = += -= *= **= /= .= %= &= |= ^= <<= >>= | 赋值运算符 |
| 左 | and | 逻辑运算符 |
| 左 | xor | 逻辑运算符 |
| 左 | 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] = $i++; // may set either index 1 or 2
?>Note:
尽管 = 比其它大多数的运算符的优先级低，PHP 仍旧允许类似如下的表达式：if (!$a = foo())，在此例中 foo() 的返回值被赋给了 $a。
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 ¶
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 expectedAnother 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!