This is a very bad idea. Project managers shouldn't work as developers and developers shouldn't work as project managers. These roles MUST be completely separate.
A project manager who's doing development tasks will not be good at these tasks and will not be good at his primary role of a project manager. A project manager just doesn't have enough time to do actual development work ON a project, and if he has enough time on his hands, this means that he's not doing his job properly.
Companies shouldn't get cheap when it comes to complete role separation.