Description:
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Now that the so called "Hacker's Guide" has been removed from the
manual[1], I suggest to add a single page with curated links to
external resources which have info about developing php-src,
extensions, SAPI etc.  This page might be added to the PH manual's
appendices.

Obviously, one such external resource would be the PHP Internals
Book[2]; there are several blogs[3][4] which may also be
worthwhile to link to.  And likely there is some up-to-date info on
zend.com, and probably much more elsewhere.

[1] <https://github.com/php/doc-en/pull/152>
[2] <http://www.phpinternalsbook.com/>
[3] <https://nikic.github.io/>
[4] <http://blog.jpauli.tech/tag/php/>