Updated PHP book

Added link to PHP security guide.
Updated PHP books
Added OS book: Operating System Concepts
Add ML book: AI, A Modern Approach
[es] Adding new Section & book: SQL
Merge pull request #816 from esparta/master

[es] Adding new Section & book: SQL
Merge pull request #815 from ScottL/master

Added 1 OS book, 1 AI book
Merge pull request #814 from virendrachandak/patch-2

Updated PHP list
Added MySQL book

Added MySQL book
Merge pull request #817 from virendrachandak/patch-2

Added MySQL book