On the Symfony blog they've shared a post giving you an "inside look" at their Call for Papers process for the SymfonyCon and SymfonyLife events. Each conference is different and has a different process for attracting speakers and selecting them once the Call for Papers has completed.

All the Symfony conferences we organize are aimed at gathering the Symfony community together to enable every community member to meet each other, share best practices and learn tips and about new features. This all happens in a convivial atmosphere that feels like a big family reunion. The highlight, of course, is listening to high quality talks. The speaker line up is very important to us and we take great care to make sure the talks we select will be appreciated and useful for the majority of conference attendees.

[...] The CFP criteria are available online and are the same for all the conferences: we're looking for the best Symfony-focused topics that are relevant to the community. We don't select speakers based on any racial, gender or physical criteria: all the talk proposals received are studied no matter who the speaker is.

They talk about what they provide to speakers, how they review the talks once the CFP is complete and some about the committee that does the selection. With the speakers they want selected they then send out the acceptance notifications and confirm schedules. The post finishes up talking about their emphasis on trying to improve the ration of female to male speakers and their code of contact.