Upgrade Centos 5 tot PHP 5.2 or 5.3 [recommended way?]

I do not see why "this sounds too good to be true".

Considerinh PHP 5.2 was released quite a few years ago, the problem is that Redhat/CentOS are really late about that (yeah, I know, "stable" and all that, which means not updating anything except for security reasons)

Happily, some people stepped up, and did the job that had to be done : create the RPM packages that so many of us needed ;-)
Only thing there might be : it won't be "offcial" packages ; so maybe not OK with the policy of your system admins ?

Still, if in doubt : before using those on your production server, you should try them on a test server (if you don't have one, a Virtual Machine will do just fine), just to be sure -- and, also, to check that your application works fine on 5.2

On the other hand, you might want to try compiling PHP by yourself : it is definitly not that hard, and it's a good thing to know :-)

You need only to add the repositories from dotdeb on the /etc/apt/sources.list file. Then use aptitude to reload the sources and check the php packages you have.

If the packages are correctly done, you should not lose any config, since configuration files are not overwritten during updates: if there's a conflict dpkg will ask what to do and you can choose to override or just save the new version with a different extension.

After that, just check the README.Debian on /usr/share/doc/ package documentation to see if there's anything else to complete the upgrade.