[PHP] How know from wich page you came from

[Rodrigo]

Hi guys I need a way to know how to know from wich page the visitor
came, something like:
 
I wanna put in a php file a switch to do a certain action if the visitor
came from a certain page, and something else if he came from a diferent
one, this way I could write, update, insert, delete or anything I need
from a single file.
 
Thanks for any kind of input,
Rodrigo