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 case-insensitive constant
$var = NULL;
Casting a variable to null using (unset) $var will not remove the variable or unset its value. It will only return a