AS3.0 is fully object oriented language, more advanced than PHP in terms of OOP. As said above, it resembles Java. You can use it as scripting engine for FLASH or FLEX or create lightweight standalone SWF files based entirely on AS3.0 code.
A few years ago I wrote a book about programming in AS3.0 for beginners. However, the book was written in Polish (unfortunately) language, which means that with the same effect I could write the book in some forgotten for over 5,000 years deceased language, which nobody knows. Now, I am rich in the experience and used it as a step forward in my development, so this set of lessons will be based on the book but now written – obviously – in English language.
The last point concerns Adobe company – they don’t support or are about to drop support for FLASH and FLEX due to some weird reasons. However, FLASH technology is still used due to its advantages, especially for games creation and advertising. HTML5 was thought to replace FLASH / FLEX but it is not that effective and not that flexible, and comes with no protection for written code. So, it seems that for coming several years FLASH / FLEX will still be used.