General developer forum

I may be missing something obvious, but I'm unable to create the simplist of filters.

I'm simplifying what I really want to do in order to make it more obvious what's happening...  I want to replace every occurance of a word (let's say "cat") within a URL linked from my moodle server with another word (let's say "dog.")  For example Topic 1 in a course has the following HTML (I've removed the space between a and href because the forum wants to translate it for me. GRRRR):
<ahref="rtsp://somesite.streams.com/mystreamfolder/cat/video1.rm">Video 1</a>

I want it to say:
<ahref="rtsp://somesite.streams.com/mystreamfolder/dog/video1.rm">Video 1</a>

I have 100 courses that each have 20 links.  I want to update just one filter every quarter and have all the animal names changed.

So here's what I did.  I followed the instructions here:
http://docs.moodle.org/en/Filters_%28developer%29
which did nothing.  Allow me to elaborate...

I made a folder called wordreplace and put the following files in it:

filter_wordreplace.php
function filter_wordreplace($text, $courseid) {
        return str_replace($CFG->filter_wordreplace_old, $CFG->filter_wordreplace_new, $text);
}

filterconfig.html
<?PHP
 echo "Swap out...<br />\n";
 echo "<input type="text" name="filter_wordreplace_old" size="60" value="$CFG->filter_wordreplace_old"><br />";
 echo "With...<br />\n";
 echo "<input type="text" name="filter_wordreplace_new" size="60" value="$CFG->filter_wordreplace_new">";
?>


I went to the filter admin area, and could not find anything there about the "wordreplace" filter.  I went rooting around in the /admin/filters.php file and figured out I probably should be using "filter.php" instead of "filter_wordreplace.php"  So I changed the filename from "filter_wordreplace.php" to "filter.php" and that allowed me to see everything on the admin screen.  I enabled the filter and put "dog" and "cat" in their respective locations, but it didn't replace the text like I wanted it to.  It just added the function (in plaintext) to text to the top of every course.  I wrapped the function in PHP tags and that got rid of the function text at the top of every course, but the function still doesn't work.

I'm at a loss.  Has anyone built a working filter in 1.6?  Can someone give me some pointers about what I might be doing wrong?