PHP运算符问题

为什么if ($a = 100 && $b = 200) {var_dump($a, $b);}输出bool(true) int(200)

<?php
if ($a = 100 && $b = 200) {
    var_dump($a, $b);
}

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老生常谈的问题了

实际上是这样运行的

php
if($a = (100 && $b = 200)){
    var_dump($a, $b);
}

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实际上的优先级是这样的：

<?php
if ($a = (100 && $b = 200)) {
    var_dump($a, $b);
}

所以$a是true。

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赋值不是比较，你可能看错了

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&&优先级比=高,所以会$a = (100 && $b = 200),在http://php.net/manual/en/language.operators.precedence.php中有一句
Although = has a lower precedence than most other operators, PHP will still allow expressions similar to the following: if (!$a = foo()), in which case the return value of foo() is put into $a.,虽然&&优先级高于=,但是200还是会被赋值给$b,由于100 && $b = 200均为真返回true,所以$a=true