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.

>>1
I read that as
Turing ascii smileys
Went "AWESOME!"
Now I'm disappointed