Summary

Writing extensions for PHP is not all that hard thanks to the myriad of tools and macros made available to you by the PHP developers. Custom extensions are a great way to add new functionality to the language, or even just to speed up certain operations that would otherwise have been too slow.
There is a trade-off between speed and usability that makes extensions the best place to put most code - while it is possible to edit the Zend Engine directly, it does limit your distribution options.
There are lots of helpful libraries that have yet to be converted into PHP extensions - pick one and give it your best shot!

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