(There was an older extension also named Parsoid which is no longer maintained or used; see Special:PermanentLink/1916875 if you want to learn about that.)

The Parsoid extension exports a REST API for the PHP port of the Parsoid wikitext parsing service. It is only required if you are running RESTBase as a caching layer in front of Parsoid.

For installation instructions, see Parsoid/PHP.