(I work at flickr) The problem is finding all the places that the language has changed and all the places you relied on the old behavior. The documented stuff

(I work at flickr) The problem is finding all the places that the language has changed and all the places you relied on the old behavior. The documented stuff was easy to find, it was the undocumented (or unintended?) changes that were hard. For example, strtotime($null) returns now in php4 but returns null in php5. QA and automated testing will only get you so far in this process. Nobody had thought to test how we display the "Date Taken" on photos from before December 14th 1901 until a member pointed out they were displaying wrong (caused by another change to strtotime() behavior). At some point you have to upgrade a subset of your servers, watch the error logs, fix the bugs as they come up and be prepared to rollback quickly. It wasn't a fun experience...