if you are in a state to restore a variable to its unassigned state you can use unset() Function
Example:

this will make the $assigned_var variabel to the default value of which type it can hold no matter what assignement it has met so far.

this function will get a variable as an argument and make the variable to default state and return nothing

Related posts:

1. testing if a variable is assigned in PHP with isset() function
2. Reassigning Variables in PHP
3. Assigning Variables in PHP
4. PHP Variables Basic