emerge world just upgraded dev-libs/mm from 1.2.0 to 1.3.0, which silently broke dev-lang/php, which was still linked to libmm.so.12, which already disappeared. The dev-libs/mm upgrade (or older mm version unmerging) should have been either blocked by the php package, or the php package should have been rebuilt automatically.
We won't be rebuilding anything automatically, that's what revdep-rebuild is for. Happens with source-based distros, live with it. ;) revdep-rebuild --library libmm.so.12