The fact that is_null throws an exception when we use it with a variable that has never been assigned a value, but not when we have assigned NULL to it, suggests that we should have a different function, say isset_strict, to determine if a variable has been assigned a NULL or non NULL value to it. In fact, a few have proposed such a function in the comments. I am not suggesting it should be a built in function. I am just pointing out that an alternative view on "is set" is in the air. 

However, I feel it is important to bring out clearly, that the current definitions of isset, is_null and NULL are very harmonious and logical. In particular isset is the opposite of is_null, we unset a variable if we assign NULL to it, we set a variable if we assign a non NULL value to it and the purpose of isset is to determine if a variable is set. It's clean and simple. 

Note that we did not need the extra "... and is not NULL", which refers to a different view.  However, it is perhaps better not to use "is set", because it might conflict with the way some people use it.

The fact that unset($v) undo the effect of $v = NULL, also reinforces this other view where $v is set after $v = NULL. Again, just pointing out that this other view is in the air, It almost feels as if phpers have adopted the concept that isset does not determine if a variable is set.

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