The special NULL value represents a variable with no value. NULL is the only possible value of type NULL.
Note: The null type was introduced in PHP 4.
A variable is considered to be null if:
it has been assigned the constant NULL.
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 keyword NULL.
$var = NULL;
Casting a variable to null will remove the variable and unset its value.