一个php运算符优先级问题

vivion32

先看一个运算符优先级表

Operator Precedence（运算符优先级）
引用
AssociativityOperatorsAdditional Information
non-associativeclone newclone and new
left[array()
non-associative++ --increment/decrement
right~ - (int) (float) (string) (array) (object) (bool) @types
non-associativeinstanceoftypes
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
left?:ternary
right= += -= *= /= .= %= &= |= ^= <<= >>= =>assignment
leftandlogical
leftxorlogical
leftorlogical
left,many uses


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&& 优先于 = 优先于 and


但是

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$a = 100 && $b = 200


按照运算符优先级规则顺序应该是

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$a = ( ( 100 && $b ) = 200 )


但是php里有规定，

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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.


所以实际效果是

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（ $a = 100 ）&& （　$b = 200　）


相当于把　&& 变成了 and

另外，引用鸟哥的一个例子

引用
最后, 顺便说一下, PHP对应于T_BOOLEAN_AND 还定义了 T_LOGICAL_AND(and) 和 T_LOGICAL_OR(or) , 这俩个的优先级都低于等号, 于是就会有了, 很多PHP入门教材示例代码中经典的:

$result = mysql_query(*)  or die(mysql_error());