Thank you for expressing an interest.
For now, the best course of action likely would be for you to start sending pull requests for (preferably small, for now) changes you have in mind, and those that are likely to be accepted and reviewed by the broader community audience. Once the change is ready to be reviewed, please ask other like-minded individuals on the list here to review the change and work with you to get it ready. With enough consensus and +1 votes in place, we'd be glad to merge the change.
Since we're short on PHP expertise, we're going to have do the development "in public" for a while until you have a solid, steady stream of changes incoming and have established a reasonable repertoire. If you keep this up at a reasonable pace, I'd be happy to call for a vote and get you maintainer access.
Best of luck!