In a new post Mark van der Velden has posted a quick (handy) example of how you can write your PHPUnit tests to be PHP version aware and only test what's needed.
I had a problem with running test cases on multiple CI environments, where one of the two runs on PHP 5.2 and the other on PHP 5.3. This basically meant that all our pretty PHP 5.3 code caused the builds to fail on the 5.2 only machine.
His technique is based on skipping tests that aren't meant for the latest version - like a test that'd only work if the server's running PHP 5.3 or greater. The key is in the PHP_VERSION constant that makes it simple to check what you're running. He also points out that you can use the @depends to achieve a similar functionality.