PHP is very easy to learn.  It is much more English like and straight-forward than Perl, although more specific to the web and therefore not as powerful for tasks outside of web development.  PHP has always seemed to be to be a simplified version of C++, if you've ever used that.

I think Dan has a great idea with using robots.txt to keep search engines out of the old forum.  It would keep the resource-hungry crawlers out of the old forum while you figure out to make the redirection work. 

I wouldn't think that eliminating inactive users would reduce the load by 2/3, which is what you needed to get to "normal" usage.  Pruning/archiving probably would have a bigger impact.

You'd have a good start.  PHP uses similar syntax and function groupings.  PHP also has a similar philosophy concerning including files and libraries in code to reduce the amount of typing you have to do.

To my knowledge, PHP doesn't support classes and is not object oriented like C++, which is the harder stuff to learn and the more powerful stuff to use.