Backporting PHP4 session management

My current employer was hired in the fall to write a backend for a website that would be used for several other sites in different markets. In the fall, their initial requirements were that it be done in PHP3 and MySQL. After the typical months of bureaucracy and poor planning, we're just now starting the project. With the release of PHP4 so near, and the size of the project, I have no doubt that PHP4 will be released months before we complete our project, but the inspired corporate mentality has decided not to let us use PHP4. Having that extraneous background information out of the way, my question is.. How feasible would it way to backport PHP4's session management to 3, either in part or in full? Matt