tl;dr: They made some bad decisions about Unicode and everyone got burned out.
They skipped 6 because 7 contains none of the Unicode changes and didn't want to confuse people.
Basically a lot of authors thought they'd get 'ahead of the game' and publish PHP6 books very early. If a 'real' PHP6 was released now, there would be confusion.
e: Actually wasn't there a blog post posted to HN suggesting Perl skip to 7 too?