$string = "Username: <-- begin --> Message <-- end -->";
$string = preg_replace("'<--[^>]*?-->'si","",$string);
$ary_chat = split(": ",$string);
echo($ary_chat[0].$ary_chat[1]);
This will turn it into an array, where 0 is the username and 1 is the message.
For removing all tags, you can use regular expressions (as seen above--ereg_replace(), preg_replace()) or strip_tags().