We've been hearing about PHP 6 since 2005, but nothing has been brought to fruition yet. In fact, the project was so plagued with problems that in 2010, it was abandoned. It almost felt like PHP would be perpetually stuck at version 5.
Except now there's talk about another major release. It could be PHP 6, or maybe PHP 7. Between developers, there's a hot debate over what the next major PHP release will be called.
The reason why this question even comes up, is that there has been a previous attempt at a new major version, which was started in 2005 and abandoned in 2010 due to difficulties in the Unicode implementation. Apart from language-integrated Unicode support, most features added for that version were integrated either in PHP 5.3 or PHP 5.4.
This previous attempt at a new major version was also developed under the name of PHP 6 and as such there are various resources referring to it, including a number of books. There is concern that there might be confusion between the abandoned previous attempt and the work that is currently happening.
If you're not familiar with what happened with the original PHP 6, you can read more its unfortunate demise here.
Given that the entire Internet was anticipating a PHP version that will never actually ship, it's probably better to stick with PHP 7.