php-packages are now available in the backports repository.
You can add the backports repository to your /etc/ipkg.conf file, so that all you need to do is to use ipkg install <packagename> just like with the base packages.
Your /etc/ipkg.conf file could end up looking something like:
src whiterussian http://downloads.openwrt.org/whiterussian/packages
src non-free http://downloads.openwrt.org/whiterussian/packages/non-free
src backports http://downloads.openwrt.org/backports/rc5
dest root /
dest opt /opt
dest ram /tmp