I'm a pentester, I was messing with a website that would fetch the results of whatever website was listed in the host header, here is an example: OOB Resource Load
You can see, the actual website I was requesting was p.example.com, but when changing the "Host" header to "www.google.com" it fetched google's homepage.
I find this functionality (whether purposeful or not) extremely interesting and would like to mess around with it on my own site.
To summarize: I'd like to embed the functionality to make a request to my personal webserver with a modified host header, then my webserver would make a request to whatever website I add in the modified host header, and it would display the result from the modified host header as the response.