The problem:

You want your WordPress post to appear in more than one category, e.g. PHP and WordPress  OK, that’s not a problem.
However, the article is primarily about WordPress but for some reason the permalink insists on it being under PHP (www.mydomain.com/php/my-post) and not WordPress (www.mydomain.com/wordpress/my-post). A little annoying ,no?!

The solution:

The old school / hack it solution would be to go into the database and manually alter the ids of the categories – WordPress will always favour the category with the lowest id. Fortunately you don’t have to do it this way, there is a nice plugin that does it very elegantly via the WP backend.

That plugin is the Hikari Category Permalink and it does just what is required:

Wordpress multiple category permalink plugin

WordPress multiple category permalink plugin

Whilst it is hasn’t been updated for over 2 years it works just fine with the current version of WordPress (3.5).