VII. PHP and Zend Engine Internals
    If you are about to begin developing PHP or Zend extensions, you need
    to prepare yourself for the programming environment provided by the
    various APIs. This part of the documentation tries to introduce the APIs
    provided by the different PHP and Zend Engine versions available.
    Since most of the information available here is somewhat outdated,
    you'll want to read various files found in the PHP source, files such
    as README.SELF-CONTAINED-EXTENSIONS and
    README.EXT_SKEL in addition to the manual.