ptkdb allows you to debug CGI scripts running on the server, by adding something to the shebang line. DDD does not. DDD requires you to essentially debug Apache, which results in debugging ALL scripts on the machine.
I've been using ptkdb to debug CGI scripts for about six months. When I saw your reference to DDD, I took a look. It's a pretty slick program. Unfortunately, if you're needing to debug something running on a web server, forget it.
That would be why.
Alternately, if you know how to get DDD to pop up a debugging window to debug a particular script running on a server (not necessarily ALL scripts on the server), please enlighten us.