The reason I updated Plesk 10 to Plesk 11 was initiated by an alert from our hosts 1and1 about a server vulnerability for ‘ISC BIND 9’ which deals with various DNS functionality within Plesk.
The version I was running was ‘BIND DNS Server 9.4.2’.
Instead I'd leave the default 5.3.3 version (at the time of this post) for Plesk to use, and assign the updated version in the service plan and hosting settings for specific accounts that need an updated version of PHP: Plesk will automatically update PHP for the version of PHP that it needs to function along with its other base packages.
So if you follow the above, you'll be keeping the base PHP install separate and functioning with fewer potential problems than with updating it to the latest PHP version that might not work well with Plesk.
I discovered pretty much the only way to do it is to install all the desired components without Plesk installed and then install Plesk post update as it is a dependency for PHP.
I've had an issue for quite a while now that I don't know how to fix.
I'm running a VPS with Ubuntu 12.04 LTS, it has the latest version of Plesk (12.0.18 Update #34).