This site has instructions and tools for building, translating, and maintaining the PHP Documentation , which is currently available in multiple languagues plus the original English.
Besides the PHP languague itself and its core features, the documentation includes material about most PHP extensions , which often could use additional contributions.
Most of the translation teams are also in need of more helping hands. If you are interessted, you should contact the relevant mailing lists. The projects and translations have separate mailing lists.