Hydrogen is a lightweight PHP 5.3+ toolkit to simplify the building of custom, dynamic web applications. Its main focus is on making webapps run ridiculously fast, with performance and high-traffic scalability being the absolute highest concern. Hydrogen has clocked in with impressive statistics, sometimes doubling the traffic that small servers with MySQL-intensive sites can handle.
Hydrogen's other main focus is portability, allowing your webapps to be installed on servers with different operating systems, SQL services, caching services, and semaphore capabilities installed without needing to change a single line of your code -- not even the database queries. Hydrogen is planned to support a wide range of these services by version 1.0. Please see the documentation link below for details