Any chance you could clarify this a bit further? I don't really understand what you're trying to achieve here!
Do you mean as soon as the swf has loaded, you want it to take a screenshot of the entire web-page??
If so, it sounds a bit unlikely to me as the swf will not be able to see anything external to itself. So it wouldn't be possible to take a screenshot of the entire web-page directly from Flash/AS3 (at least as far as I'm aware!)
If it is possible, it would probably require quite a lot of work and resources to set up. What do you intend to do with the screenshot after you've got it?
The idea is to use this image in the swf. the swf is actually an overlayer that will cover the whole page and using the screenshot we will add the illusions that the page is being torn or kicked or whatever.
Can you tell me more about the server side languages I can use? I doubt that php would be a good choice. please give me more details if possible.
About the resources, it's not necessary to have a new screenshot every time the swf is loaded, so I guess a server side script would be perfect to make a screenshot every hour lets say.
To get a screenshot of a site you would actually have to have something load the site that does render it and then take a screenshot of the site. You are correct though, PHP would probably not be the best for this task. The best to use would be something like C++ or C# depending on the server you host on. I guarantee you that it would be no easy task though.
Here is a forum thread from another site that went round and round with the question. You may be able to get something out of it.