Unless you do a full recompile from sources on the 13.04 platform, I'd recommend NOT installing any pre-packaged version of an older PHP version on a later OS. The reason is that there are many incompatibilities between major releases and you'll be UNpleasently surprised when things do not work.

In any event, its going to be a hassle taking time and effort. You'll probably be forced into recompilation and reinstallation of a number of dependent packages as well to make this work. It could require a lot of effort to keep the old version as newer support packages are released and updated by the vendor as well (i.e. more recompiles and reinstalls).

Also, keep in mind that there are problems fixed in the newer version of PHP (security issues) that are not worth backtracking to an older version.

One other suggestion might be to reinstall an OS that supports the PHP version you desire as part of the distribution.