Re: This needs to stop
" It used to be said a lot about the (lack of) ethnic diversity in pubic services: turns out if you place your job advertisements in places where they'll actually be seen by people from ethnic minorities then they do actually apply."
This applies to jobs with low barriers to entry. I would guess that expertise is required to give a talk at a conference, so advertising this conference at fried chicken shops* isn't likely to get many (useful) responses. Unless you think black/female/gay PHP developers read different technical mailing lists to white ones, this approach doesn't help.
Maybe there is some fancy way of reaching out to people that nobody has yet thought of. But as a conference organizer, finding enough women is a perennial challenge. We recently got an e-mail from a conference venue saying there were not enough women in our line-up (we were 24% women and their goal is the suspiciously round 25%). If you have any ideas where all these women are hanging around, I'd love to know.
* This isn't racist, it's a topical reference to a recent Home Office campaign.