PHP is simpler, powerful-er, and apparently faster. (probably because it isn't as all-purpose. Haven't seen too many side-by-side comparisons though, so I'm not sure how much of a performance difference exists)
As for Flash/Actionscript, I think the same general rules apply. Play around with code and you will (hopefully) figure out whats going on. I don't touch Flash much because I hate it, but
http://www.flashkit.com was suggested to me when I first
tried to make Flashes...
Hope that is marginally helpful.