PHP逻辑运算符的优先级会奇怪地影响变量分配结果 [英] PHP Logical Operators precedence affects variable assignment results strangely

问题描述

$var4 = 123;

function fn1($p1)
{
    return array('p1' => 1, 'p2' => 2);
}

if ($var1 = fn1(1) AND $var4 == 123)
{
    print_r($var1);
}

if ($var2 = fn1(1) && $var4 == 123)
{
    print_r($var2);
}

if (($var3 = fn1(1)) && $var4 == 123)
{
    print_r($var3);
}

1. 如果运行此简单脚本，它将在以下位置输出奇怪的结果: 至少对我来说！如果表达式将导致第一个的第一个输出 从函数&返回的数组分配给$ var1 变量，这是我所期望的，好吗?
2. 如果表达式将导致整数，则从第二个第二个输出 分配给$ var2变量的值为'1'，这根本不是预期的！！ 请注意，唯一更改的是逻辑运算符， 我用过'&&'而不是与"，仅此而已！
3. 如果表达式将再次产生预期的结果，则来自第三项的第三项输出 从函数&返回的数组分配给$ var3变量， 与第一个if表达式完全相同，但是请稍等:我刚刚接受了 方括号中的if表达式中的赋值语句，而 仍在使用第二个if表达式代码！

任何人都可以从技术上详细解释为什么这种奇怪的行为吗? php.net参考链接将不胜感激.

我知道&&"优先级比"AND"高，但这并不能向我解释！

解决方案

PHP:运算符的优势

&&的优先级比=高，因此在第二个if中，您将fn1(1) && $var4 == 123的值(真或假)分配给$ var2.

在第一个if中，AND的优先级低于=，因此首先进行赋值，然后比较结果.

在第三种情况下，该赋值首先发生，因为parens中的所有内容都首先得到处理.

$var4 = 123;

function fn1($p1)
{
    return array('p1' => 1, 'p2' => 2);
}

if ($var1 = fn1(1) AND $var4 == 123)
{
    print_r($var1);
}

if ($var2 = fn1(1) && $var4 == 123)
{
    print_r($var2);
}

if (($var3 = fn1(1)) && $var4 == 123)
{
    print_r($var3);
}

1. If you run this simple script it will output strange results, at least for me!! First output from first if expression will result in an array returned from the function & assigned to the $var1 variable, which is what I'm expecting, well?
2. Second output from second if expression will result in an integer '1' assigned to the $var2 variable, which is NOT expected at all!! Please note that the only changed thing is the logical operator, I've used '&&' rather than 'AND', that's all!!
3. Third output from third if expression will result again the expected array returned from the function & assigned to the $var3 variable, exactly as the first if expression, but wait: I've just embraced the assignment statement in the if expression within brackets, while still using the second if expression code!!

Can anyone explain technically -in details- why this strange behavior? php.net reference links will be appreciated.

I know that '&&' has higher precedence than 'AND' but that doesn't explains it to me!!

解决方案

PHP: Operator Precendence

&& has a higher precedence than =, so in the second if, you are assigning the value of fn1(1) && $var4 == 123 (true or false) to $var2.

In the first if, AND has a lower precedence than =, so the assignment happens first, then the result is compared.

In the third if, the assignment happens first again because everything in parens gets processed first.

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