先看一个运算符优先级表
Operator Precedence（运算符优先级）
引用
Associativity Operators Additional Information
non-associative clone new clone and new
left [ array()
non-associative ++ -- increment/decrement
right ~ - (int) (float) (string) (array) (object) (bool) @ types
non-associative instanceof types
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
left and logical
left xor logical
left or logical
left , many uses
non-associative clone new clone and new
left [ array()
non-associative ++ -- increment/decrement
right ~ - (int) (float) (string) (array) (object) (bool) @ types
non-associative instanceof types
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
left and logical
left xor logical
left or logical
left , many uses
引用
&& 优先于 = 优先于 and
但是
引用
$a = 100 && $b = 200
按照运算符优先级规则顺序应该是
引用
$a = ( ( 100 && $b ) = 200 )
但是php里有规定，
引用
Note: Although = has a lower precedence than most other operators, PHP will still allow expressions similar to the following: if (!$a = foo()), in which case the return value of foo() is put into $a.
所以实际效果是
引用
（ $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());
$result = mysql_query(*) or die(mysql_error());