In this new advisory posted on the Secunia.com site today, there's an announcement of a highly critical update for Ubuntu users for their PHP distributions.
Ubuntu has issued an update for php. This fixes some vulnerabilities, where some have unknown impacts and others can be exploited by malicious users to bypass certain security restrictions and potentially by malicious people to compromise a vulnerable system.
Check out the posting to grab the links for the packages for the various distributions (and for the different version numbers of Ubuntu). As this issue is marked as "highly critical" it's recommended that you update your installation as soon as possible.