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 cant 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 