Derick Rethans has shared some of his thoughts on how to not be evil when it comes to making changes in languages like PHP. He suggests that any backwards compatibility break should be treated with the weight it deserves and not just thrust upon users.

This is a repost of an email I sent to PHP internals as a reply to: "And since you're targetting[sic] the next major release, BC isn't an issue." This sort of blanket statements that "Backwards Compatibility is not an issue" with a new major version is extremely unwarranted. Extreme care should be taken when deciding to break Backwards Compatibility. It should not be "oh we have a major new version so we can break all the things"

He talks about the two kinds of backwards compatibility breaks: obvious things where features are removed or changed in a major way and subtle changes in how the underlying code for PHP works ("subtle changes"). He points out that most of the frustrations from users comes from the second type, making for a slower adoption rate and maybe not even adopting at all.

Can I please urge people to not take Backwards Compatibility issues so lightly. Please think really careful when you suggest to break Backwards Compatibility, it should only be considered if there is a real and important reason to do so.