How to know site url? (http_host)

Hi:

I have a hosted site with 2 different domains pointing to it, example: www.a.com and [URL=“http://www.b.com”]www.b.com

Is there anyway to read the url where the user came from in flash? I could change my flash movie according to the url of the user…

In ASP i use:

[FONT=Calibri]Request.ServerVariables("HTTP_HOST")[/FONT]

Thanks