blocks|key|4443471|text|当涉及到动态数据处理时，它们的行为是不同的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4443472|如果变量为空(+'‘)，则null合并将把该变量视为true，但速记三元运算符不会。|4443473|$a+=+NULL;
$c+=+'';

print+$a+??+'1b';
print+"\n";

print+$a+?:+'2b';
print+"\n";

print+$c+??+'1d';
print+"\n";

print+$c+?:+'2d';
print+"\n";

print+$e+??+'1f';
print+"\n";

print+$e+?:+'2f';|code-block|syntax|javascript|4443474|和输出：|4443475|1b
2b

2d
1f

Notice:+Undefined+variable:+e+in+/in/ZBAa1+on+line+21
2f|4443476|链接：https://3v4l.org/ZBAa1|offset|length|4443477|entityMap|0|LINK|mutability|MUTABLE|url|https://3v4l.org/ZBAa1^0|0|0|0|0|0|3|M|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Z|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|10|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|11|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|12|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|13|8|@]|9|@$O|14|P|15|1|16]]|A|$]]|$1|Q|3|-4|5|6|7|17|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]]]

Both of them behave differently when it comes to dynamic data handling.

If the variable is empty ( '' ) the null coalescing will treat the variable as true but the shorthand ternary operator won't. And that's something to have in mind.

<pre><code>$a = NULL;
$c = '';

print $a ?? '1b';
print "\n";

print $a ?: '2b';
print "\n";

print $c ?? '1d';
print "\n";

print $c ?: '2d';
print "\n";

print $e ?? '1f';
print "\n";

print $e ?: '2f';
</code></pre>

And the output:

<pre><code>1b
2b

2d
1f

Notice: Undefined variable: e in /in/ZBAa1 on line 21
2f
</code></pre>

Link: <a href="https://3v4l.org/ZBAa1" rel="noreferrer">https://3v4l.org/ZBAa1</a>

blocks|key|4443545|text|似乎使用??或?:都有优点和缺点。支持使用?:的是，它计算的结果是false和null，以及"“相同。缺点是，如果前面的参数为空，它将报告一个E_NOTICE。对于??，优点是没有E_NOTICE，但缺点是它不会计算false和null。在我的经验中，我看到人们开始交替使用null和false，但后来他们最终求助于修改代码，使其与使用null或false一致，但不是两者都使用。另一种方法是创建一个更复杂的三元条件：(isset($something)+or+!$something)+?+$something+:+$something_else。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4443546|以下示例说明了同时使用null和false时使用??运算符的区别：|4443547|$false+=+null;
$var+=+$false+??+"true";
echo+$var+.+"--- ";//returns:+true---

$false+=+false;
$var+=+$false+??+"true";
echo+$var+.+"--- ";+//returns:+---|code-block|syntax|javascript|4443548|但是，通过详细介绍三元运算符，我们可以使false或空字符串"“的行为如同它是null一样，而不会抛出e_notice：|4443549|$false+=+null;
$var+=+(isset($false)+or+!$false)+?+$false+:+"true";
echo+$var+.+"--- ";//returns:+---

$false+=+false;
$var+=+(isset($false)+or+!$false)+?+$false+:+"true";
echo+$var+.+"--- ";//returns:+---

$false+=+"";
$var+=+(isset($false)+or+!$false)+?+$false+:+"true";
echo+$var+.+"--- ";//returns:+---

$false+=+true;
$var+=+(isset($false)+or+!$false)+?+$false+:+"true";
echo+$var+.+"--- ";//returns:+1---|4443550|就我个人而言，我认为如果PHP的未来版本包含另一个新的运算符：:?来取代上面的语法，那就太好了。//+$var+=+$false+:?+"true";语法将对null、false和"“求值相等，而不抛出E_NOTICE...|4443551|entityMap^0|4|2|7|2|L|2|2A|2|5T|1T|0|O|2|0|0|0|0|V|2|1C|R|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]|$9|13|A|14|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|15|8|@$9|16|A|17|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|18|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|19|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|1A|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|1B|8|@$9|1C|A|1D|B|C]|$9|1E|A|1F|B|C]]|D|@]|E|$]]|$1|S|3|-4|5|6|7|1G|8|@]|D|@]|E|$]]]|T|$]]

It seems there are pros and cons to using either <code>??</code> or <code>?:</code>. The pro to using <code>?:</code> is that it evaluates false and null and "" the same. The con is that it reports an E_NOTICE if the preceding argument is null. With <code>??</code> the pro is that there is no E_NOTICE, but the con is that it does not evaluate false and null the same. In my experience, I have seen people begin using null and false interchangeably but then they eventually resort to modifying their code to be consistent with using either null or false, but not both. An alternative is to create a more elaborate ternary condition: <code>(isset($something) or !$something) ? $something : $something_else</code>.

The following is an example of the difference of using the <code>??</code> operator using both null and false: 

<pre><code>$false = null;
$var = $false ?? "true";
echo $var . "---&lt;br&gt;";//returns: true---

$false = false;
$var = $false ?? "true";
echo $var . "---&lt;br&gt;"; //returns: ---
</code></pre>

By elaborating on the ternary operator however, we can make a false or empty string "" behave as if it were a null without throwing an e_notice:

<pre><code>$false = null;
$var = (isset($false) or !$false) ? $false : "true";
echo $var . "---&lt;br&gt;";//returns: ---

$false = false;
$var = (isset($false) or !$false) ? $false : "true";
echo $var . "---&lt;br&gt;";//returns: ---

$false = "";
$var = (isset($false) or !$false) ? $false : "true";
echo $var . "---&lt;br&gt;";//returns: ---

$false = true;
$var = (isset($false) or !$false) ? $false : "true";
echo $var . "---&lt;br&gt;";//returns: 1---
</code></pre>

Personally, I think it would be really nice if a future rev of PHP included another new operator: <code>:?</code> that replaced the above syntax. ie:
<code>// $var = $false :? "true";</code> That syntax would evaluate null, false, and "" equally and not throw an E_NOTICE...

blocks|key|4443352|text|class+a
{
++++public+$a+=+'aaa';
}

$a+=+new+a();

echo+$a->a;++//+Writes+'aaa'
echo+$a->b;++//+Notice:+Undefined+property:+a::$b

echo+$a->a+??+'$a->a+does+not+exists';++//+Writes+'aaa'

//+Does+not+throw+an+error+although+$a->b+does+not+exist.
echo+$a->b+??+'$a->b+does+not+exist.';++//+Writes+$a->b+does+not+exist.

//+Does+not+throw+an+error+although+$a->b+and+also+$a->b->c+does+not+exist.
echo+$a->b->c+??+'$a->b->c+does+not+exist.';++//+Writes+$a->b->c+does+not+exist.|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|4443353|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>class a
{
 public $a = 'aaa';
}

$a = new a();

echo $a-&gt;a; // Writes 'aaa'
echo $a-&gt;b; // Notice: Undefined property: a::$b

echo $a-&gt;a ?? '$a-&gt;a does not exists'; // Writes 'aaa'

// Does not throw an error although $a-&gt;b does not exist.
echo $a-&gt;b ?? '$a-&gt;b does not exist.'; // Writes $a-&gt;b does not exist.

// Does not throw an error although $a-&gt;b and also $a-&gt;b-&gt;c does not exist.
echo $a-&gt;b-&gt;c ?? '$a-&gt;b-&gt;c does not exist.'; // Writes $a-&gt;b-&gt;c does not exist.
</code></pre>

blocks|key|4443414|text|当使用像$_GET或$_REQUEST这样的超全局变量时，你应该知道它们可能是一个空字符串。在这种特殊情况下，该示例|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4443415|$username+=+$_GET['user']+??+'nobody';|code-block|syntax|javascript|4443416|将失败，因为$username的值现在是空字符串。|4443417|因此，在使用$_GET甚至$_REQUEST时，您应该使用三元运算符，如下所示：|4443418|$username+=+(!empty($_GET['user'])?$_GET['user']:'nobody';|4443419|现在，$username的值如预期的那样是'nobody‘。|4443420|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|T|8|@]|9|@]|A|$]]|$1|K|3|L|5|D|7|U|8|@]|9|@]|A|$E|F]]|$1|M|3|N|5|6|7|V|8|@]|9|@]|A|$]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

When using the superglobals like $_GET or $_REQUEST you should be aware that they could be an empty string.
In this specal case this example

<pre><code>$username = $_GET['user'] ?? 'nobody';
</code></pre>

will fail because the value of $username now is an empty string.

So when using $_GET or even $_REQUEST you should use the ternary operator instead like this:

<pre><code>$username = (!empty($_GET['user'])?$_GET['user']:'nobody';
</code></pre>

Now the value of $username is 'nobody' as expected.

Can someone explain the differences between <a href="https://www.php.net/manual/en/language.operators.comparison.php#language.operators.comparison.ternary" rel="nofollow noreferrer">ternary operator shorthand</a> (<code>?:</code>) and <a href="https://www.php.net/manual/en/language.operators.comparison.php#language.operators.comparison.coalesce" rel="nofollow noreferrer">null coalescing operator</a> (<code>??</code>) in PHP?
When do they behave differently and when in the same way (if that even happens)?
<pre><code>$a ?: $b
</code></pre>
VS.
<pre><code>$a ?? $b
</code></pre>

PHP ternary operator vs null coalescing operator

有人能解释一下PHP中 (?:)和 (??)的区别吗？什么时候它们的行为是不同的，什么时候是相同的(如果真的发生了)？$a ?: $bVS.$a ?? $b

问PHP三元运算符与空合并运算符
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