How to fetch the Id of the previously added record

[eluser]skillfullbus[/eluser]
Hi All,
I am making a registration page wherein the data entered by the user would go into two different tables in the database.

Now the second table has User Id as a foreign key to the User table (1st table) primary key which is auto increment.

Now while adding data to the 2nd table I need the primary key of the user table (1st table). How do I get it? Pl help.

Thanks !