This are some list of changes. http://blog.tuvinh.com/one-minute-with-php6/ http://davidwalsh.name/php6 http://www.php.net/~derick/meeting-notes.html
Why are these changes will be labeled as v6 instead of 5.5 maybe? Php 5 gave us a new OOP approach that really changed something. The biggest change is the full Unicode support, but this isn't really a big change in the way PHP works.
So, why is php 6 a major release?
This is an interesting question. PHP 5.3 itself has seen the release of so many features http://www.webdigi.co.uk/blog/2009/what-is-new-in-php-53-for-php-amateurs/
But along with unicode support, there are a lot of "features" set to go which might mean why they need to call it version 6.
A lot of development practices will change and so developers need to know its a version 6.0
I think the 'Cleanup of Functionality' is a major breaker. Imagine loads of legacy PHP websites unable to run on the new PHP6.
Unicode support is definitely important. One of the reasons for me to prefer ASP.NET.
Many features now available in PHP 5.3 were planned for PHP 6. On the other hand, PHP 6 will not be fully compatible with previous versions of PHP. This legitimates a new major version.
To support unicode, a lot of internal code has been rewritten for 6. While the interface is (hopefully) mostly unchanged, the implementation has seen a lot of changes. That is bound to bring all sorts of intrinsic bugs and changed behaviour. I think this warrants the major version bump.
As noted by others in this thread, the actual reason behind it is probably more historical; A lot of the new stuff got moved from 6 to 5.3, so now 6 looks like a small jump from 5.3.
I think that the main issue is economical.
It has all to do with backwards compatibility. On a a "minor release" or "point release" (5.1 -> 5.2), you're not supposed to break backwards compatibility by removing any functionality or changing the behavior of things. This is normally a good thing as most software has many point releases over the months and years and we don't want to constantly waste time fixing newly-broken things. Regardless, quite often things are deprecated (retired but still functional) in favor of something better.
But at some point, all of those things changes, improvements, and deprecations need to be replaced by the something better. And that's when you do a "major release".
In a "major release" - which normally happen only every few years - you're allowed to break backwards compatibility. You're allowed to kill off all those deprecated functions, and you can still add new functionality. Of course, you can't surprise people with all these changes, so you have to announce them far in advance with samples, new howtos, and new documentation. It's not an easy task, but it has to be done at some point.
Is this an opportunity for new books, training, and updating applications? Yes... but so is updating desktop apps from Windows 95 to Windows XP or OS X's Panther to Tiger to Leopard.
Useful reading: http://en.wikipedia.org/wiki/Software_versioning#Change_significance
As noted by others in this thread, the actual reason behind it is probably more historical;