I presume you mean it's a hot topic if $count_comments is at least 25:
if($count_comments>=25)
{ // hot topic
$replycount='hot topic!';
}
else
{ // not a hot topic yet
$replycount='';
}
Funny name, $replycount
If you're feeling really adventurous, you can do that in one line:
$replycount = ($count_comments>=25) ? 'hot topic!' : '';
but you're probably be better served by reading up on PHP's comparison perators