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The reason for PHP-Nuke's popularity is five-fold.
1) PHP-Nuke is easy and graphic and made for the low end user who lacks knowledge of PHP/HTML.
2) PHP-Nuke was the first major portal system to be known.
3) PHP-Nuke has thousands of modules that require only a google search.
4) PHP-Nuke is updated often (what they don't realize is that is because not much time is spent on a release.)
5) PHP-Nuke has an attitude that inspires newer PHPers to use huke, and transfer of CMS is always a hassle.
Post-Nuke is like the advanced version of PHP-Nuke, your next step into the world of PHP so to speak. I however gave up on PHP-Nuke when I found how useless it was on a Windows server. PHP-Nuke is also so common that it is well-known how to hack it.
1) Post-Nuke is more open to helping developers.
2) Post-Nuke is more secure.
3) Most PHP-Nuke modules work on Post-Nuke.
However PostNuke doesn't have the graphics or the power other, lesser known CMSes pack.
Having used PHP-Nuke and then Post-Nuke for a while I found that it can suit your needs but not your wants. Many other portal systems such as Xoops, OCPortal, or PhpMyWebThings are better at giving you what you want and need.