Add Lambda PHP.

README.md

 * [Nmap](https://github.com/willdurand/nmap) - A PHP wrapper around [Nmap](http://nmap.org/).
 * [Code Mover](https://github.com/dantleech/code-mover) - A library for moving code.
 * [Iter](https://github.com/nikic/iter) - A library that provides iteration primatives using generators.
+* [Lambda PHP](https://github.com/igorw/lambda-php) - A Lambda calculus interpreter in PHP.
 * [Country List](https://github.com/umpirsky/country-list) - A list of all countries with names and ISO 3166-1 codes.
 * [PHP-GPIO](https://github.com/ronanguilloux/php-gpio) - A library for playing with the Raspberry PI's GPIO pins.