$cpanel_user = 'abc';
$cpanel_user_password = 'xyz';
$conn = mysql_connect($dbhost = 'localhost', $cpanel_user , $cpanel_user_password );

$sql = 'DROP DATABASE dbname';
$retval = mysql_query( $sql, $conn ); // works perfect

$drop_user = "DROP USER 'username'@'localhost'";
$retval = mysql_query( $drop_user , $conn );