In a recent message to the PHP internals mailing list Sara Golemon has announced the development of a PHP specification, a document formally defining the interfaces and structure of the functionality of the language. The effort is being spearheaded by a group at Facebook.

We (As in PHP) have been talking about making a spec for the PHP language for a LONG time. With PHPNG around the corner, the need for a formal spec is even more important so that we can reliably ensure that PHP.Next matches PHP 5.6's behavior as much as possible. Meanwhile, other implementations of PHP (like HHVM) should be as spec compliant as possible so that we don't see the language bifurcate. To that end, we (as in Facebook), have been putting together a formal language spec for PHP (using PHP 5.6 as the source of truth) along with an additional conformance test suite (which compliments Zend/tests).

An initial version (a "sneak peek") has already been posted providing a great start to the effort. There's already been a lot of support for the project in the community and some of the concerns around workflow and maintenance are already starting to be addressed.