I have a video that is encoded with DivX, I want to use a PHP (or other language) script on a website to detect if they have the codec, if they do play the movie, if they don't I want to tell them they need to install it and give them a link.

Can PHP do this? Can any language do this? If so can someone point me in the right direction?

The only thing I can think of that you might be able to do this with is ActiveX. PHP is a server side language so you aren't going to be able to get any information from the client other then what the client sends in it's request.

Every browser I've ever used gave me a broken plugin image with a message telling me to install something anyway, so even if it were possible, there wouldn't be much point.