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.

>>24,25
Hello, people who are raging at a person who left two years ago.

>>23,26
Hello, worthless people.