PHP: Good 4->5 transition, they almost made the Python 3 mistake with PHP 6, but sanely dropped it and the PHP 5 -> 7 seems to be going along nicely...
JS is looking a lot like the C++ story. Totally different language.
The 5 -> 7 transition was great from the point of view of someone writing PHP, but it was significantly more painful for extension writers. The only way I could update my stuff was with many hours of careful spelunking through the PHP source looking for examples of the new APIs, then many more hours of fun with valgrind working out all of the ZVAL indirection and zend_parse_parameters changes I'd missed.
I'm fine with the burden of the upgrade being transferred to users of the C API - that seemed like a very astute trade to me - but the process could've been made a lot easier with a proper upgrade guide rather than a few hasty, incomplete notes in the wiki.
JS is looking a lot like the C++ story. Totally different language.