In a recent post to his blog Dhiraj Patra introduces the iPFaces mobile PHP framework and shows how to create a simple "Hello World" kind of application.
iPFaces is a flexible solution for easy development of form-oriented network mobile applications. With the iPFaces solution, mobile devices are able to render content received from a server using their native UI components. It uses thin presentation client (must be installed on device) to render application content. Using iPFaces it is possible to build an application where users can use their device's specific component behavior and additional device features, such as location service and additional graphic components of the device (lists, pickers etc.).
He outlines the architecture of a typical application and includes a bit of PHP code to create a simple screen with a "Hello World Application" title and a "Hello World" message as well as a more complex application that uses a GPS element to figure out the distance between you and another location.