what is the new platform? did they upgrade from php4 to php5 or is it a MySQL upgrade, apache Upgrade, all 3? for starters, I would try a different connection script; ideally, your host should provide you with information on how to connect to your databases. Try this: set up all of your variables in php variables.
$host = "gatekeep.accountsupportmysql.com";
$user = "gatekeep_***"";
$pass = "****";
$database = "gatekeep_mymedia";
//connect to database
$connection = @mysql_connect($host, $user, $pass);
//select a database to work with
$selected = @mysql_select_db($database, $connection);
include this in the top of each php page that needs a connection to the db.