A great idea would be to build a social network for PHP conference attendees. There may already be one out there so excuse me if I go on a bit...
People would post up conferences, you could look at a the conference and you could see a list of people going. Maybe you could import your facebook or twitter contacts to see if you know any people who are going.
You could add social events and say you are attending them.
I think the main barrier of people going to conferences other than the financial one, is that many people don't like going into social environments where they know no-one. This isn't just isolated to PHP developers either ;) Perhaps some kind of gateway where conference attendees could interact before attending would be a great way of encouraging people to network *before* the conference.