PHP shouldn't be used to handle data like that. Python or something even more low level like c running on a computer or server would do.
That unfortunately was what I concluded after writing this post, and was out of the question to implement.
I have worked around this however. The application I was developing was to be used internally to provide a simple web interface for webcam monitoring. As I could not stream ASF directly through PHP due to several reasons I instead built a small script that would get a "snapshot" from the webcam every second, and save it to a publicly viewable directory. I just used plain HTML to show that image, and a simple meta refresh to update it every second. So, a 1fps feed. Not perfect, but considering the client was myself and the webcam was monitoring my cats, it does the job perfectly.