Anybody who programs in PHP can be a contributing member of the community that develops and deploys it; the task of deploying PHP, documentation and associated websites is a never ending one. With every release, or release candidate comes a wave of work, which takes a lot of organization and co-ordination.
With the introduction of release managers comes a smoother release process, but help is still needed: testing release candidates, finding and squashing bugs in tests, documentation, and sources.
Someone hoping to become involved in the maintenance and development of the source should be experienced in all of the areas mentioned above, as this creates a strong team; everyone knows how every other part of the project works.
You will also need experience in C programming as PHP is written entirely in C.