I don't understand this line:
$_conn = @mysql_conn($Host,$User,$Password,$DBName);
Is "mysql_conn" your own personal function? Because it's not a PHP function. The correct PHP function is mysql_connect, and the (optional) 4th parameter is not a database name. It's a boolean flag for new_link. So, this line SHOULD read:
$conn = @mysql_connect($Host,$User,$Password);
mysql_select_db($DBName);
If changing that does not fix your problem, I'm not sure what's going on -- can you use that username/password to view the database in phpMyAdmin?