PHP is in the main repository.
$ apt-cache policy php5
500 http://nl.archive.ubuntu.com/ubuntu/ precise-updates/main amd64 Packages
500 http://security.ubuntu.com/ubuntu/ precise-security/main amd64 Packages
500 http://nl.archive.ubuntu.com/ubuntu/ precise/main amd64 Packages
and supported for 5 years:
$ apt-cache show php5 | grep ^Supported
Canonical's definition for software in this repository is:
The main component contains applications that are free software, can
be freely redistributed and are fully supported by the Ubuntu team.
This includes the most popular and most reliable open-source
applications available, many of which are included by default when you
install Ubuntu. Software in main includes a hand-selected list of
applications that the Ubuntu developers, community and users feel are
most important, and that the Ubuntu security and distribution team are
willing to support. When you install software from the main component,
you are assured that the software will come with security updates and
that commercial technical support is available from Canonical.
So, yes, the policy is to provide security updates to PHP for five years, independently of what PHP is doing. This is not unique to Canonical, but also Red Hat, Debian and other distributions backport security patches for newer versions.
I don't think that point releases of Ubuntu LTS will upgrade to PHP 5.4 - it may be available as a separate package though. See also What's the deal with point releases in LTS versions?