代码之间有什么区别

Question

我在PHP中有以下几行代码：

$a = $b && $c || $d;

和

$a = $b AND $c || $d;

和

$a = $b && $c OR $d;

和

$a = $b AND $c OR $d;

每一行都使用不同的运算符，因此它是不同的，如果是，那有什么区别？ 它是如何执行的？

Answer 1

执行这些运算符的顺序不同。

Operator Precedence

// The result of the expression (true && false) is assigned to $g
// Acts like: ($g = (true && false))
$g = true && false;

// The constant true is assigned to $h and then false is ignored
// Acts like: (($h = true) and false)
$h = true and false;

Answer 2

以下是运营商优先顺序：

了解更多信息：http://us2.php.net/manual/en/language.operators.precedence.php

Answer 3

作为short-circuit, they are the same.＆amp;＆amp; means和, || means或`，它们的行为相同：

// foo() will never get called as those operators are short-circuit
$a = (false && foo());
$b = (true  || foo());
$c = (false and foo());
$d = (true  or  foo());

另一方面，||的优先级高于or，

// The result of the expression (false || true) is assigned to $e
// Acts like: ($e = (false || true))
$e = false || true;

// The constant false is assigned to $f and then true is ignored
// Acts like: (($f = false) or true)
$f = false or true;

与&&和and相同：

// The result of the expression (true && false) is assigned to $g
// Acts like: ($g = (true && false))
$g = true && false;

// The constant true is assigned to $h and then false is ignored
// Acts like: (($h = true) and false)
$h = true and false;