gettype and unknown type in php

I've just started to practice with PHP built-in gettype() and its return value. This function is capable to return testing result such as boolean, integer, unknown type, etc. But among 开发者_开发百科those testing result, there's one caught my eyes: unknown type.

After reading gettype() and trying to find some reference here, i can not get any.

So, the question is what kind of type can be categorized as unknown type? Is it possible or am I just missing reading something?

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Here is one unknown type for you:

$f = fopen('somefile.txt','r');
echo gettype($f); // resource
fclose($f); 
echo gettype($f); // unknown

Basically, whenever a resource pointer is closed, the variable holding the handle will point to an unknown resource. Another example would be with GD'S imagecreate/imagedestroy.

Note: as of PHP 7.2, this no longer holds true. gettype will return resource (closed) then.

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According to the PHP source code it's the "default" case in the switch statement for that function. My guess is that is is there way of handling an internal error.