One way that you can keep installing packages past end-of-life is to add newer or older (possibly archived) PPAs (Personal Package Archives) to suit the type of software that you need to install or keep installed. This will actually allow you to use the apt-get install/update functionality that you are used to, but it use a non-end-of-life repository.
To add a PPA, use:
add-apt-repository ppa:<ppa-id> "ppa-id" should be replaced by the name of the PPA you are looking to add, in this case I believe it would be a PPA (in place of Ubuntu's default for 13.04) that still has the xdebug updates you are looking for. One that I have found, but have not used personally is: [2]
ppa:ondrej/php5 I recommend that you personally evaluate it for your own use, because I do know the specific use you are looking for. There is also a version that supports PHP 5.4 if that is more useful to you: [2]
ppa:ondrej/php5-oldstable In general, the PPA is a community maintained repository, usually for one piece of software (php5-xdebug) or a collection of software (php related tools might be an example). Often, the PPA will maintain the newest version of the software, or even unstable development builds with new features, but there are also PPAs that maintain access to old versions of software that Ubuntu as a release does not by default. It is important to remember that you are providing direct access to install software onto your computer, so you should make sure you trust any PPAs you are going to add to your Ubuntu installation. [3]
Short of upgrading to the current release, I think that this solution will be your best bet for continuing to install packages.
Sources:
[1] Adding a PPA