[Dygear]

Well first off, you'll need to connect to the MySQL database that the forum is using, you can get the forum's database credentials from the config.php file within the includes directory. From there, connecting to the database, and then selecting the users table. There you would want to check against the username and password. But keep in mind that the password has a salt. So you'll MD5 (I'm pretty sure that's what they are using here) the password the user supplys, and then take the MD5'd password and MD5 that with the salt concated on to the end of the string. If that matches the information in the password column for that username, then they they should be successfully logged in.

Now I'm not sure if it's MD5 or SHA1, MD5 is kinda out of vogue due to a slight problem with hash collisions, so must people updated to SHA1, but even SHA1 has it's collision problems.