sending usernames to php script

hi when a user logs in they are taken to a page where they can play mp3 and
download them but for the user can download a mp3 my script checks a data base for the download limit all users start off with 5 downloads per month .I need amember to store the logged in user name to a variable so that mysql knows where to look

Code:

$q=mysql_query("UPDATE downloads SET mdr = '4' WHERE username = '$y'");