在 PHP 中 is_null($x) 与 $x === null
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is_null($x) vs $x === null in PHP
提问by Explosion Pills
Possible Duplicate:
What's the difference between is_null($var) and ($var === null)?
PHP has two (that I know of, and three if you count isset()) methods to determine if a value is null: is_null()and === null. I have heard, but not confirmed, that === nullis faster, but in a code review someone strongly suggested that I use is_null()instead as it is specifically designed for the null-evaluation purpose. He also started talking about math or something.
PHP 有两个（我知道，如果你算的话，三个isset()）方法来确定一个值是否为空：is_null()和=== null。我听说过，但没有证实，这=== null更快，但在代码中，有人强烈建议我改用is_null()它，因为它是专为空评估目的而设计的。他也开始谈论数学什么的。
Anyway, the fact that is_null()is apparently slower also leads me to believe that it's doing more than === nulldoes and is probably preferred. Is there any reason to use one or the other? Is one always preferred? What about isset()?
无论如何，is_null()显然更慢的事实也让我相信它做得更多=== null并且可能是首选。有什么理由使用其中之一吗？总是首选吗？怎么样isset()？
As an addendum to possibly not get this question closed, what about isset()vs. is_null()? It seems that all isset()will do is suppress the notice, so unless you actually wanta notice for an undefined variable, any reason to use is_null()instead? How about if you know the variable is initialized at the time?
作为可能无法解决此问题的附录，isset()vs.is_null()呢？似乎所有isset()要做的就是取消通知，所以除非你真的想要一个未定义变量的通知，否则有什么理由is_null()改用？如果您知道该变量当时已初始化呢？
Finally, is there any mathematical reason to prefer is_null()over === null? Something about null not being comparable?
最后，是否有任何数学理由，更喜欢is_null()了=== null？关于 null 的东西没有可比性？
回答by NikiC
There is absolutely nodifference in functionality between is_nulland === null.
有绝对没有在两者之间的功能差异is_null和=== null。
The only difference is that is_nullis a function and thus
唯一的区别是它is_null是一个函数，因此
- is marginally slower (function call overhead)
- can be used as a callback, e.g.
array_map('is_null', $array).
- 稍慢（函数调用开销）
- 可以用作回调，例如
array_map('is_null', $array)。
Personally, I use null ===whenever I can, as it is more consistent with false ===and true ===checks.
就个人而言，我null ===尽可能使用，因为它更符合false ===和true ===检查。
If you want, you can check the code: is_identical_function(===) and php_is_type(is_null) do the same thing for the IS_NULLcase.
如果你愿意，你可以检查代码：is_identical_function( ===) 和php_is_type( is_null) 对IS_NULL案例做同样的事情。
The related isset()language construct checks whether the variable actually exists before doing the nullcheck. So isset($undefinedVar)will not throw a notice.
相关isset()语言构造在进行null检查之前检查变量是否实际存在。所以isset($undefinedVar)不会扔通知。
Also note that isset()may sometimes return trueeven though the value is null- this is the case when it is used on an overloaded object, i.e. if the object defines an offsetExists/__issetmethod that returns trueeven if the offset is null(this is actually quite common, because people use array_key_existsin offsetExists/__isset).
另请注意，即使值是isset()有时也可能返回- 这是在重载对象上使用时的情况，即如果对象定义了/方法，即使偏移量也是返回（这实际上很常见，因为人们使用在/ ）truenulloffsetExists__issettruenullarray_key_existsoffsetExists__isset
回答by jprofitt
As stated by others, there is a time difference between using ===and is_null(). Did some quick testing and got these results:
正如其他人所说，使用===和之间存在时间差异is_null()。做了一些快速测试，得到了这些结果：
<?php
//checking with ===
$a = array();
$time = microtime(true);
for($i=0;$i<10000;$i++) {
if($a[$i] === null) {
//do nothing
}
}
echo 'Testing with === ', microtime(true) - $time, "\n";
//checking with is_null()
$time = microtime(true);
for($i=0;$i<10000;$i++) {
if(is_null($a[$i])) {
//do nothing
}
}
echo 'Testing with is_null() ', microtime(true) - $time;
?>
Gives the results
给出结果
Testing with === 0.0090668201446533
Testing with is_null() 0.013684034347534
使用 === 0.0090668201446533 进行测试
使用 is_null() 0.013684034347534 进行测试
回答by mAu
I'm not able to say wether it's better to use is_nullor === null. But be aware when using isseton arrays.
我不能说使用is_null或更好=== null。但是isset在数组上使用时要注意。
$a = array('foo' => null);
var_dump(isset($a['foo'])); // false
var_dump(is_null($a['foo'])); // true
var_dump(array_key_exists('foo', $a)); // true
回答by Marc B
They all have their places, though only isset() will avoid undefined variable warnings:
它们都有自己的位置，但只有 isset() 会避免未定义的变量警告：
$ php -a
Interactive shell
php > var_dump(is_null($a));
PHP Notice: Undefined variable: a in php shell code on line 1
bool(true)
php > var_dump($a === null);
PHP Notice: Undefined variable: a in php shell code on line 1
bool(true)
php > var_dump(isset($a));
bool(false)
php >
回答by ThiefMaster
You need isset()if the variable is possibly not defined. It returns false when the variable is not defined or === null(yes, it's thatugly). Only isset()and empty()do not raise an E_NOTICE if the variable or array element does not exist.
您需要isset()如果变量可能是没有定义。当没有定义的变量或返回false === null（是的，这是丑陋的）。只有isset()和empty()如果变量或数组元素不存在不引发E_NOTICE。
There is not really a difference between is_nulland === null. I think ===is much nicer but when you e.g. need to use call_user_funcfor some dubious reason, you'd have to use is_null.
is_null和之间没有真正的区别=== null。我认为===要好得多，但是当您例如call_user_func出于某种可疑原因需要使用时，您必须使用is_null.
回答by PiTheNumber
===and is_nullis the same.
===并且is_null是一样的。
According to this commentis_nullis only 250ns slower. I think because a function is slower than an operator.
根据此评论is_null仅慢 250ns。我认为是因为函数比运算符慢。
回答by Shakti Singh
The PHP documentationhas a good discussion and experiments on is_null, === null, isset. Especially read the comment section.
在PHP文档有一个很好的讨论和实验is_null, === null, isset。尤其是看评论区。
