Funnily, there is no PHP 6. While work had begun for this version, it was later abandoned due to some major issues regarding Unicode handling, as far as I know. Most of the features already implemented in PHP 6 were shipped in PHP 5.4 and they continued with PHP 7 in order to avoid confusion.

IIRC this release is so different from the planned 6 release they thought it would be confusing to people who bought/read those PHP6 books.