Which will make me more money in the long run, Flash or PHP?

You can make a lot more money with PHP. The main issue with Flash is that search engines don't like it and people are starting to avoid it. It's mainly used for banners now, and not for professional websites who try to rank high on search engines.

 

And not many websites even use it for banners. It's usually for art style sites or portfolios, not a great proportion of websites.

 

Actionscript 3.0 is pretty similar to javascript, and the two can work together on a web site : ) If you already know js, Actionscript could be very quick to learn.

http://www.bitstorm.org/edwin/en/php/ <--interesting read. Most of the problems he complains about should be fixed in PHP6.