There you go, the sql query returned 0 results in the first place, which is why echo $replies['replycount']; gives you nothing.
Are you sure there is a thread with threadid 715925 present ? Seems like an awefully large number, you must have a huge board
[edit] : ok checked your site ( assuming this is the site you are using the code for ).
Seems this thread id is not valid, could have been a deleted thread :
http://www.thefinalfantasy.com/forum...d.php?t=715925