How many PHP versions there exist out there?

I keep hearing about PHP6 all the time but apparently this version has been in the works for quite some time and there is still no official time table on its release.

“A new major version has been under development alongside PHP 5 for several years. This version was originally planned to be released as PHP 6 as a result of its significant changes, which included plans for full Unicode support. However, Unicode support took developers much longer to implement than originally thought, and the decision was made in March 2010^[12]to move the project to a branch, with features still under development moved to a trunk.“
(http://en.wikipedia.org/wiki/PHP)

To get a list of all PHP versions that ever existed, please click one the following links below:

PHP Releases History
http://php.net/releases/index.php

Reference
http://en.wikipedia.org/wiki/PHP

WordPress coding standards
http://codex.wordpress.org/WordPress_Coding_Standards

Drupal Coding Standards
http://drupal.org/coding-standards

PHP coding standards
http://pear.php.net/manual/en/standards.php http://www.evolt.org/node/60247

Indent Style 
http://en.wikipedia.org/wiki/Indent_style

Microsoft 
http://msdn.microsoft.com/en-us/library/czefa0ke(vs.71).aspx