blocks|key|3478660|text|在第一个示例中：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3478661|//Notice:+Undefined+index:+test+
++++if($_GET['test']+===+'somestring'+&&+!empty($_GET['test']))+{
}|code-block|syntax|javascript|3478662|您尝试先访问变量的值，然后再检查它是否存在，这基本上就是第二个示例所做的事情。在检查到它不存在后，它退出条件，而不测试第二部分。|3478663|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

In the first example: 

<pre><code>//Notice: Undefined index: test 
 if($_GET['test'] === 'somestring' &amp;&amp; !empty($_GET['test'])) {
}
</code></pre>

you are trying to access the value of the variable first, before checking if it exists, which is what the second example is basically doing. After checking that it does not exist, it exits the conditional without testing the second part.

blocks|key|3478681|text|if(!empty($_GET['test'])+&&+$_GET['test']+===+'somestring')+{|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|3478682|因为您使用的是&&，所以如果任何条件为false，则不会检查进一步的条件。条件是按照编写的顺序进行检查的。因为在这里，第一个条件!empty($_GET['test'])是false，所以下一个条件不求值，因此没有警告。|unstyled|offset|length|style|CODE|3478683|entityMap^0|0|7|2|J|5|1S|L|2E|5|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|N|8|@$G|O|H|P|I|J]|$G|Q|H|R|I|J]|$G|S|H|T|I|J]|$G|U|H|V|I|J]]|9|@]|A|$]]|$1|K|3|-4|5|F|7|W|8|@]|9|@]|A|$]]]|L|$]]

<pre><code>if(!empty($_GET['test']) &amp;&amp; $_GET['test'] === 'somestring') {
</code></pre>

Because you're using <code>&amp;&amp;</code>, if any condition is <code>false</code>, further conditions are not checked. Conditions are checked in the order they're written. Since here, first condition <code>!empty($_GET['test'])</code> is <code>false</code>, the next one does not evaluate hence no warning.

blocks|key|5348159|text|简单地说，这可以归结为operator+precedence。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|5348160|文档显示&&是从左到右计算的，这意味着首先计算子句的左侧部分。在您的示例中，该子句将导致一个警告。如果颠倒了子句顺序，empty()检查将返回false，然后逻辑将执行short+circuit并停止求值。|style|CODE|5348161|entityMap|0|LINK|mutability|MUTABLE|url|https://secure.php.net/manual/en/language.operators.precedence.php|1|https://secure.php.net/manual/en/language.operators.logical.php^0|B|J|0|0|4|2|1N|7|2C|D|1|0^^$0|@$1|2|3|4|5|6|7|R|8|@]|9|@$A|S|B|T|1|U]]|C|$]]|$1|D|3|E|5|6|7|V|8|@$A|W|B|X|F|G]|$A|Y|B|Z|F|G]]|9|@$A|10|B|11|1|12]]|C|$]]|$1|H|3|-4|5|6|7|13|8|@]|9|@]|C|$]]]|I|$J|$5|K|L|M|C|$N|O]]|P|$5|K|L|M|C|$N|Q]]]]

Simply put, it comes down to <a href="https://secure.php.net/manual/en/language.operators.precedence.php" rel="nofollow noreferrer">operator precedence</a>. 
The documentation shows that <code>&amp;&amp;</code> is evaluated left-to-right, which means the left part of the clause is evaluated first. In your case, that clause results in a warning. If you reversed your clause order, the <code>empty()</code> check would return false, then the logic would <a href="https://secure.php.net/manual/en/language.operators.logical.php" rel="nofollow noreferrer">short circuit</a> and stop evaluating.

Just curious why this would trigger a warning. 
Note: query string param "test" is not included in the URL

<pre><code>//Notice: Undefined index: test 
if($_GET['test'] === 'somestring' &amp;&amp; !empty($_GET['test'])) {
}

//Valid - No Warning. Why is this Valid? Param 'test' was never given
if(!empty($_GET['test']) &amp;&amp; $_GET['test'] === 'somestring') {
}
</code></pre>

Is this because PHP evaluates from LEFT to RIGHT? And not the whole condition?

PHP && operators

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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只是好奇为什么这会触发警告。注意: URL中未包含查询字符串参数test//Notice: Undefined index: test if($_GET['test'] === 'somestring' && !empty($_GET['test'])) {}//Valid - No Warning.  Why is ...

问PHP &&运算符
EN

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