PHP 5.6 to PHP7? But what happened to PHP6?
PHP 6 was supposed to add native unicode support to PHP. The decision to use UTF-16 internally was made early on in the process (2005). At the start of work on PHP 6, UTF-8 was not widely supported but during development, the industry started to standardize on this. Widespread adoption of this more forgiving encoding helped with a lot of the problems that PHP 6 was trying to solve and made the objectives less relevant. By 2009/2010, it had started to become apparent that PHP 6 was never going to happen.
There's a great discussion of what really happened with PHP6 and how we ended up with PHP7 instead and you can listen to the podcast at the link above to hear PHP internal developers Sara Golemon, Derick Rethans, Sean Coates and Andrei Zmievski discuss this in detail.