So I still have a spare 1&1 VServer running a slightly outdated CentOS 5.4 or so which I would like to use for site previews and tests. Among other things, I'm exploring the possibilities of a simplified lib for Semantic Web development, which involves some of the newer features of PHP 5.3 in combination with JSON-based data processing (and possibly SQLite). Unfortunately, my VServer didn't provide either, and the server initialization panel doesn't offer any current OS, possibly due to the bundled Plesk.
Although I did manage to upgrade either CentOS or PHP via yum, each time I ended up with a broken Plesk. Day-long story shorter: I eventually found a nice and simple guide at webtatic.com that worked just fine. The trick is to keep the CentOS as-is (Plesk apparently has issues with certain versions of SSL) and to update PHP with 5.3 packages that still use the pre-5.3 naming scheme (i.e. "php-common" instead of "php53-common" etc.). This way you won't get module and dependency conflicts; the new PHP is just handled as a basic update :)
Here is a quick summary of the commands needed (via webtatic):
rpm -Uvh http://repo.webtatic.com/yum/centos/5/latest.rpm yum --enablerepo=webtatic update phpAnd that's it. The only thing still missing is SQLite, which I didn't manage to get to work. The PDO extension is there, but PHP itself was initially configured as '--without-sqlite3'. For SQLite experiments, I will probably try JiffyBox, which I played with a little yesterday and which looks very promising.
Update: SQLite is now working, too.