Do you have support for php in your use line in make.conf? Look at /usr/portage/profiles/use.desc and see what applies to you. I did find this little tidbit:
php - Include support for php
There is a newuse option. You may want to try emerge -p --newuse php after you add that to your USE line in make.conf. I think that works. It should recompile the packages that needs to enable php support. If it looks OK, remove the -p option, pretend ya know.
Never used php but it may help.