ResolutionYes, Parallels Plesk Panel supports mysql5 and php5. More detailed description provided below:
This support means that there are no point in Parallels Plesk code which conflict with them. In general if you replace mysql4 with mysql5, it does not affect Plesk itself, all the SQL requests Plesk sends to it from any Plesk's place will be processed without problems. There are no dependencies which do not allow mysql5 be installed.
As for PHP, note first that we talk about PHP used by hosted web sites, it is loaded by client copy of apache as module (Plesk panel itself uses separate apache server copy with other compiled-in PHP). If you look into apache configs, you will find there that directives for both php4 and php5 modules are always included. If you replace php4 module with php5 and restart apache, your sites and PHP scripts continue working without problems.
Also neither mysql nor php are part of Plesk. Parallels does not build its own mysql and php packages, as well as Paralles does not compile and provide openssl, perl, xinetd and a lot of other system applications used by Plesk. Plesk uses standard services which are installed on the system. If mysql/php are found on the system where Plesk in installed, Plesk will use them. If these installations are not found, Plesk will download and install standard packages of OS vendor from autoinstall server.
The same can be said about Parallels Virtuozzo Containers installation. Virtuozzo OS templates and application templates are only made by Parallels as a copy of standard packages from OS vendors.
In general Plesk is tested on and oriented to standard OS installations with standard packages from OS vendor. If mysql/php v5 are available from OS vendor, for example in SuSE 10.1 or FedoraCore 5, then it is tested with them. If outdated FedoraCore 1 provides mysql3, then Plesk build for this OS is tested with it. Plesk code is almost identical for any OS and there are no global problems with mysql5 and php5 at 8.x, both of them are supported and should work w/o problems.
As for mysql5/php5 installations, there are no specific recommendations or steps because all the steps are identical to any other application update (a-la recommendation "use `rpm -U` for RedHat-like OSes"). Get package updates from OS vendor and use them. If OS vendor does not provide such packages, then mysql/php v5 you try to find can only be provided by third party. There are hundred custom builds can be found in the world, for example well known http://www.atomicrocketturtle.com is really great source for such updates for many years. Although Parallels cannot officially guarantee anything for third-party updates, if you get a custom package which fits your OS environment - you will not have problems with it. The most of problems which may appear with them are usually related to dependencies to third or system applications and can be solved accordingly. The advanced admins may even compile from sources.
If you find the problem, please report to firstname.lastname@example.org problem description: what you do, where and what kind of error happens plus system details: OS, Plesk version/build, mysql/php builds, where they were taken and so on. If it is found that this is really Plesk issue, it will be corrected in further updates.
Plesk Control Panel fully supports MySQL 5 since Plesk 8.1 version.
When php is upgraded to php5 on Debian, the Plesk panel goes down
How do I upgrade PHP to version 5 on Debian server with Plesk installed?
Is it possible to simultaneously use php4 and php5 for web applications?