In a new post to the Developer Tutorials blog Akash Mehta takes a look at migrating PHP4 applications up to the more recent versions of PHP5.
PHP 5 supports most of the legacy syntax features of PHP 4. Most code written for PHP 4 should function fine under PHP 5, and a comprehensive test suite could check this. However, many of the backwards-incompatible changes in PHP 5 were in regard to language quirks, and quite a few hacks rely on these in order to function.
He points out some of these hacks including changes to the object model and updates to the way variables are handled (breaking many "bad use" cases). He also points out the changes made in the latest Windows binaries as well as some of the new reserved keywords that could conflict with pre-existing code in your application.