is_null($x) vs $x === null in PHP
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What's the difference between is_null($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 === null is 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.
Anyway, the fact that is_null() is apparently slower also leads me to believe that it's doing more than === null does and is probably preferred. Is there any reason to use one or the other? Is one always preferred? What about 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 want a notice for an undefined variable, any reason to use is_null() instead? How about if you know the variable is initialized at the time?
Finally, is there any mathematical reason to prefer is_null() over === null? Something about null not being comparable?
Answer
There is absolutely no difference in functionality between is_null and === null.
The only difference is that is_null is a function and thus
- is marginally slower (function call overhead)
- can be used as a callback, e.g.
array_map('is_null', $array).
Personally, I use null === whenever I can, as it is more consistent with false === and true === checks.
If you want, you can check the code: is_identical_function (===) and php_is_type (is_null) do the same thing for the IS_NULL case.
The related isset() language construct checks whether the variable actually exists before doing the null check. So isset($undefinedVar) will not throw a notice.
Also note that isset() may sometimes return true even 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/__isset method that returns true even if the offset is null (this is actually quite common, because people use array_key_exists in offsetExists/__isset).