An interesting history of PHP and why they're skipping version 6.

Here is a brief summary of the reasons outlined supporting the decision to release the next version as PHP 7:

  • PHP6 already existed, and there are plenty of numbers available (so it's easy to make a new version number).
  • It could confuse people since PHP6 was a widely known project, and this release is unrelated.
  • There's lots of PHP6 information on the web which does not apply to this release.

http://halls-of-valhalla.org/beta/news/from-php-5-to-7,146/