A mas na serveri subor db.php? Mas spravne vytvorenu databazu? (tabulka "ratings" ktora ma stlpce "avg(rating)" a "idclanku" ? Mas v databaze nejaky riadok ktory ma "idclanku" = 15? Nemalo by tam byt "echo $row['avg(rating)']"?
priklady napr. http://de2.php.net/mysql_fetch_array
P.S. mas tu databazu otvorenu? Vid priklady...