There 2 problems with FreeBSD.
1. The FreeBSd instalation of plesk is very very monolitic ( all packs are pre compiled like in pkgs) and have a lot of glue and dependencies under /usr/local/psa*
2. Probably you can compile you php with mcript, xlst or something and try change for the instaled by plesk, but i never tried this
for example to install Zope I opted to compile and use one "domain" python over the use the distributed (old ) by plesk or use one wide server.
tip tip tip tip .. try use the most recent build of FreeBSD ( the 4.11 probably is the LAST 4.x branch release and the 5.4 is RC now) because this some ports or have some problems to compile under 4.11 (php+apache2+mcript+sablotron) for example.
* probably run very well under jail besides every thing is under this directory...