I recently came across an extension to a popular CMS that would not install on an Ubuntu 12.04 server, because PHP version 5.3 reached end of live. I was however under the impression that all security upgrades would be backported into Ubuntu 12.04 until its end of live.
I checked this on the changelog of the package: http://changelogs.ubuntu.com/changelogs/pool/main/p/php5/php5_5.3.2-1ubuntu4.28/changelog however I see that updates have ended on 28 Oct 2014, whereas many security updates were made to PHP 5.4 since then that I would have expected to be backdated.
On the other hand it may not be feasible to backport from PHP 5.4.
I'm not looking for a way to install the extension, but on advice if Ubuntu 12.04 with all security updates applied is vulnerable to rcent PHp security issues.