I don't think it does what you are looking for. That will always return a 1, because the SQL is going to return 1 row: "Count" that counts all the records where `topic_id` = '".$_GET['topic'].
You can do 1 of 2 things change the code to :
PHP:
$find_replies = mysql_num_rows(mysql_query("SELECT * FROM `forum_replies` WHERE `topic_id` = '".$_GET['topic']."'"));
or
PHP:
$row = mysql_fetch_row(mysql_query("SELECT COUNT(id) FROM `forum_replies` WHERE `topic_id` = '".$_GET['topic']."'"));
$find_replies = $row[0];