Lack of a native Unicode support was one of the biggest PHP drawbacks. In 2005, developers decided to change this. The newly formed project was designed to provide a native Unicode support to PHP. To achieve this goal, developers decided to use the ICU library (International Components for Unicode). Another significant feature of the project was representing UTF-16 text strings. It was planned to release the project as version 6.0, but we can see only PHP 5.3 and PHP 5.4, both without Unicode integration. As of 2015, there is an ongoing work on a new major version of PHP – PHP 7. The PHP 6 with its unicode experiment had never been released, so the name was abandoned.
UPD (23.06.15): PHP 7 – release date, performance, features, changes
UPD (13.07.15): Interview with Dmitry Stogov
UPD (25.08.15): PHP 7 on Magento EE
UPD (27.08.15): Deprecated functionality which will be removed in PHP 7
UPD (01.09.15): PHP 7.0.0 RC 1
UPD (07.09.2015): PHP 7.0.0 RC2 has been released. PHP 7 will be available on the 12th of November.
UPD (15.09.15): PHP 7 and WordPress, Return types, Scalar types, Threading for PHP, PHP7 Reference, php7cc
UPD (22.09.15): Backwards compatibility issues, PHP 7 screencasts, and a new overview
UPD (29.09.15): PHP 7 tutorial
UPD (26.10.2015): The Acquisition of Zend and PhpStorm 9
UPD (07.11.2015): PHP 7 Hidden Features
UPD (24.11.15) PHP 7 Docker Image for Magento 2
UPD (24.11.15) PHP 7 vs HHVM
UPD (01.12.15): PHP 7 RC 8 has been released
UPD (03.12.15): PHP 7 HAS BEEN RELEASED!!!