http://username:password@example.com/rss.php

id  username  rss_key
1   user_a    49f0bad29968
2   user_b    1f2414c23a7d
3   user_c    9bc46e8e52ad

http://example.com/rss.php?Key=1f2414c23a7d

<?php

$GetKey = addslashes($_GET['Key']);

//Other Rules --- Example: if(empty($GetKey)) { echo "error"; exit(); }

include("connect.php"); //Your connection file

include("session.php"); //Your session file

$Username = $User['username']; //in session file

$Match = Mysqli_Fetch_Array(Mysqli_Query($con, "SELECT u.rss_key AS 'RSSKey' WHERE user_table_name AS u WHERE u.username='".$Username."'");

if($Match['RSSKey'] !== $GetKey)
{

//Stop page
exit();

}
else{

//Your RSS Code...

}

?>