Added Functional Programming in PHP book

README.md

 * [Securing PHP: Core Concepts](https://leanpub.com/securingphp-coreconcepts) - A book about common security terms and practices for PHP by Chris Cornutt.
 * [Modernising Legacy Applications in PHP](https://leanpub.com/mlaphp) - A book about modernising legacy PHP applications by Paul M. Jones.
 * [Modern PHP New Features and Good Practices](http://shop.oreilly.com/product/0636920033868.do) - A book about new PHP features and best practices by Josh Lockhart.
+* [Functional Programming in PHP](http://www.phparch.com/books/functional-programming-in-php/) - This book will show you how to leverage these new PHP5.3+ features by understanding functional programming principles
 
 ## Other Books
 *Books related to general computing and web development.*