php-amphtml is a generated library out of the main amphtml repository. It does not try to validate correct syntax, but provides the developer with shorthand functions and classes to handle the amp tags.

Also it keep track of the dependencies and inject them accordingly in the header of the document. This method can also be called directly.

The library comes with AmpDocument.php which uses Less as its CSS generator. As AMP documents only can have one block of CSS it make sense to have something like this. Again, this is more of a proof of concept.

Be aware that the library is dependent on updates from xtazii/amp-library to get new tags supported. The current file is over 1 year old.