A Lithium PHP developer is responsible for creating, testing, and maintaining web applications using the Lithium PHP framework. They utilize this lightweight, fast, flexible, and secure framework to build scalable and high performance applications. Their tasks include writing clean, well-structured, and efficient code, debugging programs, integrating data storage solutions, and ensuring strong optimization and functionality. They also collaborate with the front-end team to integrate user-facing elements with server-side logic. Furthermore, they often contribute to the development of all server-side logic, definition and maintenance of the central database, and ensuring high performance and responsiveness to requests from the front-end.