You should also follow some sub-inclusions from these files; for
example, the ones relating to the Zend executor, the PHP
initialization file support, and such. After reading these files,
take the time to navigate around the package a little to see the
interdependencies of all files and modules - how they relate to
each other and especially how they make use of each other. This
also helps you to adapt to the coding style in which PHP is
authored. To extend PHP, you should quickly adapt to this style.