Of course, extensions can also be implemented
directly in the Zend engine. This strategy is good if you
need a change in the language behavior or require special
functions to be built directly into the language core. In
general, however, modifications to the Zend engine should be
avoided. Changes here result in incompatibilities with the
rest of the world, and hardly anyone will ever adapt to
specially patched Zend engines. Modifications can't be
detached from the main PHP sources and are overridden with
the next update using the "official" source repositories.
Therefore, this method is generally considered bad practice
and, due to its rarity, is not covered in this book.