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;
See also the functions is_null() and unset().