Zend Engine 1 is the internal engine used by PHP for the entire version 4 release line. It is no longer considered active, but PHP 4 is still in widespread use, so the old ZE1 documentation is preserved here exactly as it was.
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 Since most of the information available here is somewhat outdated, you'll want to read various files found in the PHP source, files such as README.SELF-CONTAINED-EXTENSIONS and README.EXT_SKEL in addition to the manual.