PHP中"或"的奇怪方式

Question

PHP or是一个奇怪的关键字.这是一个让我困惑的代码片段:

echo 0 or 1; // prints 1

$foo = (0 or 1);
echo $foo; // prints 1

$foo = 0 or 1;
echo $foo; // prints 0 for some reason

为什么最后一个打印0而不是1？

Answer 1

这是因为运营商的优先级不同.在第三种情况下,首先处理分配.它将被解释为:

($foo = 0) or 1;

该||运营商有不同的优先级.如果你使用

 $foo = 0 ||1;

它会像你期望的那样工作.

请参阅逻辑运算符手册

Answer 2

不,我不会,那是因为运营商优先:

 $foo = 0 or 1;
 // is same as
 ($foo = 0) or 1;
 // because or has lower precedence than =

 $foo = 0 || 1;
 // is same as
 $foo = (0 || 1);
 // because || has higher precedence than = 

 // where is this useful? here:
 $result = mysql_query() or die(mysql_error());
 // displays error on failed mysql_query.
 // I don't like it, but it's okay for debugging whilst development.