Hey, as it is i don't think flex is as readily updated as the other possible languages... Nor is the documentation as extensive as some other languages.
If i were doing what you said you wanted to do then i'd probably go about it in PHP using mySQL (as you've mentionned)... Since you have experience with Apache it shouldn't be hard to get to grips with either, and the knowledge of HTML&CSS should help a fair bit.
If you do want to go down that route then the best thing i can advise is using the PHP tutorials
here.
Having said that it's worth knowing what your apps are going to focus on? Or what you intend to do with them?
Cheers,
Jamey