On The Nerdery's blog today there's a new post that shows you a way to help test your code for PHP 5.4 and 5.5 - via PHP_CodeSniffer - as well as mentioning some of the changes that came with these versions.

Do you want to know if your code is compatible with that latest version of PHP? Do you need to upgrade your existing PHP website to a newer version of PHP? Do you want to see if your legacy PHP code is using any features that have been removed or deprecated in later versions of PHP? [...] ou need to migrate your existing code to ensure nothing will break when the upgrade occurs. How can you accomplish that?

They start with the list of differences (and backward incompatible changes) in the newer versions and point to the migration pages for each. Then they get into the use of PHP_CodeSniffer and some additional rules provided by Win Godden for checking the compatibility. With these added in, you can call the "phpcs" checking tool with the "PHPCompatibility" standard and check out the results. An example of what you might see on an older application is included in the post.