> I made some assumptions unfortunately.

Yes, been there, done that, have several t-shirts.

Live and learn. That's one mistake you won't be making again (for a while, at least). But it still sucks to be in this situation.

> Now if he makes me also have to support PHP4, I'll hang myself.

Please don't do that. However if visual basic is mentioned then you might have to go through with that, but I'd suggest hanging the client instead. It's rude, not good for business but it drives home the point ;)

In my experience, with some man-to-man (or whatever the applicable genders) talks you should be able to work it out, if the customer can be brought to your side of the table then you've solved it. One way of achieving that is to move to their side of the table, and make a list of the consequences, and have them agree - preferably in writing, an email will do - that they take responsibility for any consequences of doing it 'their way', and that any work over and beyond the agreed upon will be billed separately.

That might make them a lot more amendable to reason.