[kh99]

I know what you mean now, you just want to put something as a parameter that can't be guessed so people can't cheat. You could hash the user id (maybe concatenating it with some "secret" string value), but the only way to get back the userid would be to calculate the has value for each id until you find one that matches. Or you could do something like is mentioned above. You don't have to add a column to the user table, you could make a new table and just use any random value as a id into the table (this is how activation and password changes work, using the useractivation table. Maybe you could figure out a way to use that same table).

Or maybe you just wanted to know the name of the php function? It's md5().