I have a dedicated server with Ubuntu 8.04, and really need to upgrade php to 5.3 version in order to deploy a new version of the system.

This version of php is the default one in ubuntu 10.04, so I considered upgrading the OS, but after trying that, I lost my plesk installation, which annoyed my client.

I tried adding the dotdeb.org repositories, but don't know why, after running an apt-get upgrade, I get this error:

# apt-get upgrade
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages have been kept back:
  libapache2-mod-php5 php5 php5-cgi php5-cli php5-common php5-curl php5-gd php5-imap php5-mysql php5-sqlite php5-xsl
0 upgraded, 0 newly installed, 0 to remove and 11 not upgraded.

Any idea why is this happenning?

Or do you know any alternative method (except compiling my own binaries) to upgrade php or update ubuntu without loosing plesk?

Thanks!