There's a quick tip on the Utah PHP Users Group site today about how to get PHP5 working with your Gentoo system:

For those of us who use Gentoo and want all the power/speed/features/bragging rights of using PHP5, you know it can be tricky since the packages are hard masked for now.

For the impatient in all of us, here's a really really short summarized set of instructions on how to install it with your current setup.

Through the use of unpacking the package, recompiling, and merging things back together, you can quickly and easily get PHP5 working with the packages provided...