IIRC, the initial development was in the mid 2000s but it was abandoned several years later because they couldn't get Unicode support working correctly. By the time it was abandoned there was already a lot of material referencing it like books and conference talks. So when the PHP team decided to do a new major version most of the planned PHP 6 features were already added to 5.3 and 5.4 so they thought it best to just skip 6 to avoid confusion.