How strong it is largely depends on your own requirements, and the job at hand.
What you could do is list the requirements that matter to you, for example, documentation, ease of debugging, community support, vendor support, ease of deployment etc. For each language you want to compare, score how each one does on these requirements.
If you score Actionscript 3 the highest, you'll have answered your own question :)
However, one of your requirements is "can develop server-side code for HTML generation like PHP", and you'll find Actionscript scoring rather low there. While a limited server-side actionscript is available in Flash Media Server, it is more for providing services to client-side Flash applications than delivering HTML.