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 smooth release process, but still help is needed: testing release candidates, finding and squishing bugs in tests, documentation, and sources.
Someone hoping to become involved in the maintanence and development of the source should be experienced in all of the areas mentioned above, 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.
Below are some useful links for developers: