User Database integration

[*****Sniper]

so I should start from functions.php and altering the fetch_userinfo()

I have been changing login.php, line

if ($bbuserinfo = $DB_site->query_first("SELECT userid, usergroupid, membergroupids, username, password, salt FROM " . TABLE_PREFIX . "user WHERE username = '" . addslashes(htmlspecialchars_uni($username)) . "'"))
{

I do have to change this as well correct? even after changing the fetch_userinfo() function!?
if
one last thing... what is "iif" its not in any php manual! is it "if and only if"??