note 121296 deleted from function.isset by cmb

Note Submitter: dominic dot mayers at meditationstudies dot org ---- The comment http://php.net/manual/en/function.is-null.php#80409 brings the interesting point that is_null will throw an error if the variable was not set at all. So, we can distinguish a variable that is unset through a NULL assignment and a variable that is simply not set at all. However, the returned value under is_null is the same in both cases, only the error message differs. We can define another function that determines if the variable is "set," including "set" by a NULL assignment. It's a different concept, different definitions of isset, etc. Mixing these different concepts can be confusing. In the current definitions for isset, is_null, NULL, etc., the extra "and is not NULL" in the definition of isset is redundant, because is_null is the opposite of isset. We should not mix that with the other concept mentioned above.