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In a new post on his blog, Wez Furlong shares some of the materials from a presentation he gave at the PHP Con West back in 2003 on the topic of "Extending PHP".
While poking around the disks in my linux box, I found my materials for a session on extending PHP that I originally gave as a 3 hour tutorial at PHP{Con West 2003.
Sadly, I seem to have lost the working C code (libares bindings for PHP), but all the relevant parts can be found in the comprehensive PDF I made from the slides: Extending PHP Slides (PDF). The content is based on PHP 4, but should still be applicable to PHP 5.
The talk [PDF] covers the creation of a sample DNS resolving library to extend the functionality already in PHP, and includes some of the code examples (in PHP) to show how it all works. This is followed by the C code to build the extension, defining each of the functions, and geting it up and running.