Pico is a stupidly simple, blazing fast, flat file CMS. See http://picocms.org/ for more info.
Requires PHP 5.3+
Download composer and run it with install option.
$ curl -sS https://getcomposer.org/installer | php
$ php composer.phar install
The easiest way to Pico is using the built-in web server on PHP.
$ php -S 0.0.0.0:8080 ./
Pico will be accessible from http://localhost:8080.
Visit the Pico Wiki for plugins, themes, etc...