Description:
------------
At the moment isset call return true only when variable is set and it is not 
null.

But there is no way (or I do not find it so far), how to just test variable for 
existance. So it will be perfect if there will be some other call (for eg: exist) 
or some parametr how to get this behaviour
More in example. 

Test script:
---------------
$a = null;
$testOnlyExistence = true;

echo isset($b); // prints false;
echo isset($a); // prints false;
echo isset($a, $testOnlyExistence); // prints true;
echo isset($b, $testOnlyExistence); // prints false;
// or
echo exist($a); // prints true;
echo exist($b); // prints false;