This is not entirely unlike that. Somewhere along the way, everybody realized Perl 6 was not the path forward from Perl 5, and so Perl 6 never really took the place of Perl 5, just like PHP6 never took the place of PHP 5. It looks like a pretty similar scenario, if you squint, I think.
It makes little difference for Perl that this experiment in a next generation version of the language is still alive and going its own way. The result is the same...people kept moving forward with version 5, and if it needs a new version, it can't use version 6 because 6 was already used for that other experiment (that failed to take the place of Perl 5).
The primary goal of PHP6 was to implement full unicode support and drop mbstring. However, that took much longer than expected and multiple major features ended up getting backported to 5.3, 5.4 and 5.6; to the point that PHP 7 became the new feature version while PHP6 was worked on. They eventually gave up and PHP7 was released with unicode support built fairly deep, but basic string types and the like still being byte arrays.
PHP7 was PHP6, without the native and full unicode coverage requirement.
"after DirectX 3 was released, Microsoft began developing versions 4 and 5 at the same time. Version 4 was to be a shorter-term release with small features, whereas version 5 would be a more substantial release. The lack of interest from game developers in the features stated for DirectX 4 resulted in it being shelved, and the corpus of documents that already distinguished the two new versions resulted in Microsoft choosing to not re-use version 4 to describe features intended for version 5."
For those that can remember back that far, MacOS also jumped from 5 to 7. (7 was the first version that supported running multiple applications at once.. not counting the MultiFinder hack that came before it by a few years.)
It certainly did not. System 6 (including MultiFinder if you had enough RAM to support it ;) was active for several years in the early 90s before System 7 came out. The label "MacOS", meanwhile, didn't show up until a few years after that.
Doh, yes, system 6 was current for quite a while. I wonder what other thing from that era I confused it with. I swear something around that time skipped a 6.
Solaris skipped from 2.6 to 7. UnixWare also jumped from 2.x to 7. HP/UX skippped from 3.x to 6.x. In perhaps the strangest version jump of all time, Darwin jumped from 1.4.1 to 5.1, as part of Mac OS X 10.1.1.
It seems Unix vendors really don't like low version numbers. Of course, Windows famously jumped from 3.x to 95, so it's not like they're any better.
Windows 95 was Windows 4.0. There are several parts of Windows 95 that even indicated this in a user-visible way.
Microsoft was surprisingly good at not skipping numbers, until the jump from Windows 8.1 to Windows 10, which also lead to the NT kernel jumping from 6.x to 10.
Microsoft had to skip 9 because programmers are lazy.
A lot of programs refused to work if there was a `9` in the version name.
That was of course because those programs needed something newer than Windows 95 or Windows 98. So they just did a string search for `9` in the version name.
There may be a few programs that wouldn't work on `8.9` or `10.9` if they were released for the same reason.