Added PHP section to spanish version

Include the PHP section for the spansih reference books, then added the
"Manual de PHP".
Fix the C++ link (free-programming-books-es.md)

The c++ contextual link did not work.
Revert "Fix the C++ link (free-programming-books-es.md)"

This reverts commit aa26bdabe34dd46c4a148a11ac41009bd7d35b25.
Fix the C++ link (free-programming-books-es.md)

The c++ contextual link did not work.
Merge pull request #528 from cmeraz/master

Added PHP section to spanish version (free-programming-books.md)