You mean, generate a list of requirements based on an analysis of your source code?
While in theory, that might be possible, I don't think such a solution exists. I think there is no way than analyzing your code by hand, with the PHP manual very close by.
Do you use GD? Then you need PHP with the GD module. Do you need to create GIF images with GD? Then you need GD, but not between versions 1.6 and (I think) 1.8. Do you use PDO? Then you need PHP > 5.1.0. And so on and so on.
In short, I'm afraid think this is going to be a manual process. Manual also as in "PHP manual" - the User Contributed Notes to each function and method are a gem, and any common cross-platform problems are usually noted there somewhere.
While you can trust that PHP x.y.z has a defined set of functions and behaviour, be sure to test well before you declare something suitable to run on a different server. IIS's support of PHP is way better now, I'm told, but the last time a ported a big PHP application over to IIS, it took me three days to work around all the mysterious bugs.