This is an HTML page we can use to demonstrate the Request/Response Cycle.
Because it has a ".htm" suffix, the server simply reads it and sends the data stored in the file to your brower. P.S. Check out the sweet pre tag - it respects newlines and white space. It is great for debugging PHP code.
Make sure to play with your browser's developer console.
Note: You can look at the source code for these pages on Github.