how do you set the $port for a Mysql

I have a different port number for the mySQL connection

$db_url = "mysql://username:Password@127.0.0.1:3306/database";
$db_prefix = '';

i set it this way which allows me to get to the logon screen but i think that the parse[url] is then striping it and not letting me get any further?

Where would i set the $port ?

I found in the database.mysql.inc this line
// Allow for non-standard MySQL port.
if (isset($url['port']))

how do i change it to a custom port?