PHP is backwards compatible almost to a fault, but that doesn't mean you can't use new libraries or frameworks that hide those warts away from you, and that's a MUCH better choice in my opinion than making a significant percentage of the internet incompatible with the latest PHP so that your function arguments for a handful of old function calls can all look the same...
It's not the end of the world, and it is getting better every day, but having to spend the better part of a decade dealing with multiple "main" versions of a language is not something that I want any other languages I use repeating.
And if that means needing to lookup if the function is urlencode or url_encode, well i'm happy to pay the price.