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.