turning ascii smilies into smiley.gif PHP

Does this look right?
$comment = str_replace(":D", "<img src='myimage.gif' alt=':D' />");

It will be for the comment system seen here.
http://www.plentyoftorrents.com/flsh/animation/princess-1

Also would this replace any :D that is already posted or just when someone makes a post after its put in.

It wont work no matter what I do. :( I should have just used a forum under the flashes.
Can u find what im doing wrong?
heres the sauce
http://www.plentyoftorrents.com/flsh/animation/comments/sauce/comments_show.phps

http://www.plentyoftorrents.com/flsh/animation/comments/sauce/comments_process.phps

includes folder
http://www.plentyoftorrents.com/flsh/animation/comments/sauce/includes/clean_input.phps
(should I be taking out this? is this getting rid of html?

http://www.plentyoftorrents.com/flsh/animation/comments/sauce/includes/comments-config.phps

It looks like something keeps stripping my html code even if i disable the filters in the config file. :S