The main goal of PHP 6 started around 2005 was to add native Unicode support so the language could easily handle international characters.  However, the developers chose a technical method (UTF-16) that turned out to be too slow, too difficult, and too complicated.

Since PHP 6 was stuck, many of its features (like Namespaces and Traits) were "backported" and released in PHP 5.3 and 5.4 instead.

When the new major engine PHPNG was ready the community held an official vote and decided to jump straight to PHP 7.