blocks|key|1482945|text|卸下!。你不想否定这个表达式。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1482946|$friendid+=+isset($_GET['friendid'])+?+$_GET['friendid']+:+'empty';|code-block|syntax|javascript|1482947|entityMap^0|2|1|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

Remove the <code>!</code>. You don't want to negate the expression.

<pre><code>$friendid = isset($_GET['friendid']) ? $_GET['friendid'] : 'empty';
</code></pre>

blocks|key|1483173|text|从PHP+7's+release开始，您可以使用空合并操作符(双"?")为此：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1483174|$var+=+$array["key"]+??+"default-value";
//+which+is+synonymous+to:
$var+=+isset($array["key"])+?+$array["key"]+:+"default-value";|code-block|syntax|javascript|1483175|在PHP+5.3%2B中，如果你检查的只是一个"truthy“值，你可以使用"Elvis运算符”(注意这不会检查isset)。|1483176|$var+=+$value+?:+"default-value";
//+which+is+synonymous+to:
$var+=+$value+?+$value+:+"default-value";|1483177|entityMap|0|LINK|mutability|MUTABLE|url|https://wiki.php.net/rfc/php7timeline|1|http://www.php.net/manual/en/language.types.boolean.php#language.types.boolean.casting^0|1|F|0|0|0|M|6|1|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@$A|X|B|Y|1|Z]]|C|$]]|$1|D|3|E|5|F|7|10|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|11|8|@]|9|@$A|12|B|13|1|14]]|C|$]]|$1|K|3|L|5|F|7|15|8|@]|9|@]|C|$G|H]]|$1|M|3|-4|5|6|7|16|8|@]|9|@]|C|$]]]|N|$O|$5|P|Q|R|C|$S|T]]|U|$5|P|Q|R|C|$S|V]]]]

As of <a href="https://wiki.php.net/rfc/php7timeline">PHP 7's release</a>, you can use the null-coalescing operator (double "?") for this:

<pre><code>$var = $array["key"] ?? "default-value";
// which is synonymous to:
$var = isset($array["key"]) ? $array["key"] : "default-value";
</code></pre>

In PHP 5.3+, if all you are checking on is a "<a href="http://www.php.net/manual/en/language.types.boolean.php#language.types.boolean.casting">truthy</a>" value, you can use the "Elvis operator" (note that this does not check isset).

<pre><code>$var = $value ?: "default-value";
// which is synonymous to:
$var = $value ? $value : "default-value";
</code></pre>

blocks|key|1483094|text|如果你是懒惰和危险的，你可以使用error+control+operator+@和ternary+operator的缩写。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1483095|$friendid+=+@$_GET['friendid']?:+'empty';|code-block|syntax|javascript|1483096|entityMap|0|LINK|mutability|MUTABLE|url|http://www.php.net/manual/en/language.operators.errorcontrol.php|1|http://www.php.net/manual/en/language.operators.comparison.php#language.operators.comparison.ternary^0|G|O|0|15|G|1|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@$A|T|B|U|1|V]|$A|W|B|X|1|Y]]|C|$]]|$1|D|3|E|5|F|7|Z|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|10|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]|Q|$5|L|M|N|C|$O|R]]]]

If you're lazy and risky, you can use <a href="http://www.php.net/manual/en/language.operators.errorcontrol.php" rel="noreferrer">error control operator @</a> and short form of <a href="http://www.php.net/manual/en/language.operators.comparison.php#language.operators.comparison.ternary" rel="noreferrer">ternary operator</a>.

<pre><code>$friendid = @$_GET['friendid']?: 'empty';
</code></pre>

blocks|key|1268461|text|当前您使用的是三元运算符：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1268462|$friendid+=+(!isset($_GET['friendid']))+?+$_GET['friendid']+:+'empty';|code-block|syntax|javascript|1268463|将其分解为一条if-else语句，如下所示：|offset|length|style|CODE|1268464|if(!isset($_GET['friendid']))
+++$friendid+=+$_GET['friendid'];
else
+++$friendid+=+'empty';|1268465|看看if语句中到底发生了什么：|1268466|!isset($_GET['friendid'])|1268467|注意感叹号(!)在isset函数前面。这是另一种说法，“与之相反”。您在这里要做的是检查$_GET['friendid']中是否已经设置了值。如果是这样的话，$friendid应该承担起这个价值。|1268468|但实际上，它会崩溃，因为$_GET['friendid']甚至都不存在。你不能接受不存在的东西的价值。|1268469|从头开始，您已经为$_GET['friendid']设置了一个值，因此第一个if条件现在为false，并将其传递给else选项。|1268470|在这种情况下，将$friendid变量的值设置为empty。|1268471|您想要的是删除感叹号，然后$friendid的值将采用$_GET['friendid']的值(如果先前已设置)。|1268472|entityMap^0|0|0|7|7|0|0|2|2|0|0|9|5|18|H|27|9|0|C|H|0|9|H|12|2|1L|4|0|8|9|O|5|0|D|9|R|H|0^^$0|@$1|2|3|4|5|6|7|14|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|15|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|16|8|@$I|17|J|18|K|L]]|9|@]|A|$]]|$1|M|3|N|5|D|7|19|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|1A|8|@$I|1B|J|1C|K|L]]|9|@]|A|$]]|$1|Q|3|R|5|D|7|1D|8|@]|9|@]|A|$E|F]]|$1|S|3|T|5|6|7|1E|8|@$I|1F|J|1G|K|L]|$I|1H|J|1I|K|L]|$I|1J|J|1K|K|L]]|9|@]|A|$]]|$1|U|3|V|5|6|7|1L|8|@$I|1M|J|1N|K|L]]|9|@]|A|$]]|$1|W|3|X|5|6|7|1O|8|@$I|1P|J|1Q|K|L]|$I|1R|J|1S|K|L]|$I|1T|J|1U|K|L]]|9|@]|A|$]]|$1|Y|3|Z|5|6|7|1V|8|@$I|1W|J|1X|K|L]|$I|1Y|J|1Z|K|L]]|9|@]|A|$]]|$1|10|3|11|5|6|7|20|8|@$I|21|J|22|K|L]|$I|23|J|24|K|L]]|9|@]|A|$]]|$1|12|3|-4|5|6|7|25|8|@]|9|@]|A|$]]]|13|$]]

Currently you're working with the ternary operator:

<pre><code>$friendid = (!isset($_GET['friendid'])) ? $_GET['friendid'] : 'empty';
</code></pre>

Break it down to an <code>if-else</code> statement and it looks like this:

<pre><code>if(!isset($_GET['friendid']))
 $friendid = $_GET['friendid'];
else
 $friendid = 'empty';
</code></pre>

Look at what's really happening in the <code>if</code> statement:

<pre><code>!isset($_GET['friendid'])
</code></pre>

Note the exclamation mark (!) in front of the <code>isset</code> function. It's another way to say, "the opposite of". What you're doing here is checking that there is no value already set in <code>$_GET['friendid']</code>. And if so, <code>$friendid</code> should take on that value. 

But really, it would break since <code>$_GET['friendid']</code> doesn't even exist. And you can't take the value of something that isn't there.

Taking it from the start, you have set a value for <code>$_GET['friendid']</code>, so that first <code>if</code> condition is now false and passes it on to the <code>else</code> option. 

In this case, set the value of the <code>$friendid</code> variable to <code>empty</code>.

What you want is to remove the exclamation and then the value of <code>$friendid</code> will take on the value of <code>$_GET['friendid']</code> if it has been previously set.

blocks|key|1483043|text|这个问题的最佳解决方案是empty()，也就是说，如果你还需要“检查空字符串”。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1483044|$friendid+=+empty($_GET['friendid'])+?+'empty'+:+$_GET['friendid'];|code-block|syntax|javascript|1483045|empty()不仅检查是否设置了变量，而且如果向它提供了任何可以被视为“空”的东西，例如空字符串、空数组、整数0、布尔值false等，则还会返回false。|1483046|entityMap|0|LINK|mutability|MUTABLE|url|http://be2.php.net/manual/en/function.empty.php|1^0|C|7|0|0|0|0|7|1|0^^$0|@$1|2|3|4|5|6|7|T|8|@]|9|@$A|U|B|V|1|W]]|C|$]]|$1|D|3|E|5|F|7|X|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|Y|8|@]|9|@$A|Z|B|10|1|11]]|C|$]]|$1|K|3|-4|5|6|7|12|8|@]|9|@]|C|$]]]|L|$M|$5|N|O|P|C|$Q|R]]|S|$5|N|O|P|C|$Q|R]]]]

The best solution for this question, i.e. if you also need to 'check for the empty string', is <a href="http://be2.php.net/manual/en/function.empty.php" rel="nofollow">empty()</a>. 

<pre><code>$friendid = empty($_GET['friendid']) ? 'empty' : $_GET['friendid'];
</code></pre>

<a href="http://be2.php.net/manual/en/function.empty.php" rel="nofollow">empty()</a> not only checks whether the variable is set, but additionally returns false if it is fed anything that could be considered 'empty', such as an empty string, empty array, the integer 0, boolean false, ...

blocks|key|1268691|text|我在if条件下使用Null+coalescing+operator操作符，如下所示|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1268692|if($myArr['user']+??+false){|code-block|syntax|javascript|1268693|这相当于|1268694|if(isset($myArr['user'])+&&+$myArr['user']){|1268695|entityMap|0|LINK|mutability|MUTABLE|url|http://php.net/manual/en/migration70.new-features.php#migration70.new-features.null-coalesce-op^0|9|O|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@$A|V|B|W|1|X]]|C|$]]|$1|D|3|E|5|F|7|Y|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|Z|8|@]|9|@]|C|$]]|$1|K|3|L|5|F|7|10|8|@]|9|@]|C|$G|H]]|$1|M|3|-4|5|6|7|11|8|@]|9|@]|C|$]]]|N|$O|$5|P|Q|R|C|$S|T]]]]

I am using <a href="http://php.net/manual/en/migration70.new-features.php#migration70.new-features.null-coalesce-op" rel="nofollow noreferrer">Null coalescing operator</a> operator in if condition like this

<pre><code>if($myArr['user'] ?? false){
</code></pre>

Which is equivalent to 

<pre><code>if(isset($myArr['user']) &amp;&amp; $myArr['user']){
</code></pre>

blocks|key|1483140|text|好吧，我可能一直有一个类似的问题，因为我不熟悉！情况就像jasondavis那样。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1483141|有点困惑，但发现没有！比如..。isset($avar)与!isset($avar)相比可以产生很大的差异。|1483142|所以有了！在适当的位置，更多的是表示是，如在|1483143|++++since+$_GET['friendid']+=+55;+has+been+initialized...
++++++++++++++++++++++++++++++++tell+me+'no'+-+the+opposite+-+that+it+hasn't+and+set+it+to+empty.
++++++++++++++$friendid+=+(!isset($_GET['friendid']))+?+$_GET['friendid']+:+'empty';

++++where+not+having+the+!+tells+me+yes+it+has+something+in+it,+leave+it+be.

+++++++++++++++$friendid+=+(!isset($_GET['friendid']))+?+$_GET['friendid']+:+'empty';|code-block|syntax|javascript|1483144|与if+A$="“then....的混淆程度要小得多。去做吧。(对于PHP，如果是$A="“的话)。|1483145|我发现这种将字符串和变量都用作字符串的做法有时令人望而生畏。即使在困惑中，我也能真正理解为什么.这只会让我有点难以理解。|1483146|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|S|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|T|8|@]|9|@]|A|$I|J]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|V|8|@]|9|@]|A|$]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

Okay, I may have been having a similar issue not being familiar with the ! situation as jasondavis had.

Kind of confusing but finding out not having the ! as in... isset($avar) compared to !isset($avar) can make quite the difference.

So with the ! in place, is more stating a YES as in

<pre><code> since $_GET['friendid'] = 55; has been initialized...
 tell me 'no' - the opposite - that it hasn't and set it to empty.
 $friendid = (!isset($_GET['friendid'])) ? $_GET['friendid'] : 'empty';

 where not having the ! tells me yes it has something in it, leave it be.

 $friendid = (!isset($_GET['friendid'])) ? $_GET['friendid'] : 'empty';
</code></pre>

Was far less confusing with if A$="" then.... work it. ( or if $A="" for those of PHP ).

I find this use of strings and variables all as strings to be very daunting at times. Even through the confusion, I can actually understand why... just makes things a tad difficult to grasp for me.

This below does not seem to work how I would expect it, event though $_GET['friendid'] = 55 it is returning NULL

<pre><code>&lt;?PHP

$_GET['friendid'] = 55;

$friendid = (!isset($_GET['friendid'])) ? $_GET['friendid'] : 'empty';

echo $friendid;
exit;

?&gt;
</code></pre>

Simple PHP isset test

下面的代码似乎并不像我期望的那样工作，虽然$_GET‘’friendid‘= 55返回NULL<?PHP$_GET['friendid'] = 55;$friendid = (!isset($_GET['friendid'])) ? $_GET['friendid'] : 'empty';echo $friendid;exit;?>

问简单的PHP isset测试
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回答 7

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问简单的PHP isset测试EN

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