How to lock and update data between two table in mysql

[david lin]

Hi,

I have two mysql tables which are user and gift.
I will to do these processes in my program:
----------
mysql_query("SELECT user_points FROM users where userid='peter' FOR UPDATE");
mysql_query("SELECT gift_number, gift_points FROM gifts where product='xbox' FOR UPDATE");

if($gift_number > 0 && $gift_points > $user_points)
{
mysql_query("UPDATE user SET user_points=user_points-$gift_points WHERE userid='peter'");
mysql_query("UPDATE gifts SET gift_number=gift_number-1 WHERE product='xbox'");
**Problem 1** Is the procedure right or wrong?
**Problem 2** How to unlock these tables?
} else {
**Problem 3** How to unlock theses two tables in the else condition ? Does anyone know?
}
------------------
Thank you very much.