Comment 2 for bug 886423

It looks like your apt sources got out of sync somehow. Oneiric has 5.3.6-13ubuntu3.1 in the release pocket, and 5.3.6-13ubuntu3.2 in updates and security.

I am also able to install php5-mysql without problems on an up to date 11.10 system, so I believe this is most likely local problems to your system. Possibly try 'apt-get update' and also check 'apt-cache policy php5-mysql' to find out what source that old version is coming from.