Read my question thoroughly before responding, I know there’s a site called wonderfl.net
I‘ve got the Flex SDK 4 on my Mac and I found a way to compile AS3 into SWF files using Flex's mxmlc compiler in Xcode, so I wondered, would it be possible to do this sort of simply online? Using for example a language I'm familiar with, PHP?
I thought it’d be a thing that would be interesting to use for a website, or like some private projects.
You have the available tools to do so. You can write the AS3 content posted from the web into files, use PHP's exec function to run mxmlc, then send the resulting .SWF file to the client using PHP's readfile function. You'd just have to make sure mxmlc was present on the web server running the PHP.