End-user applications are provisioned in cloud settings using PaaS. Portability is key to the success of end-user application providers because it helps avoid vendor lock-in. It makes the components deployment of the same end-user application across multiple PaaS possible. It also enables them to move end-user applications from PaaS to PaaS with minimal adaptation efforts. This survey discusses and evaluates the approaches proposed so far within the standardization bodies, research projects, and academia for enabling end-user applications portability in PaaS. It also identifies the trajectory of research on the topic. An illustrative use case is described in order to identify evaluation requirements, and then the state of the art is reviewed in light of the requirements. The identified research directions are related to application executables, deployment descriptors, and operations, and a comprehensive set of management operations in the PaaS.