NULL value represents a variable with no value.
NULL is the
only possible value of type NULL.
A variable is considered to be null if:
it has been assigned the constant
it has not been set to any value yet.
it has been unset().
There is only one value of type null, and that is the
$var = NULL;
Casting a variable to null using (unset) $var
will not remove the variable or unset its value.
It will only return a