Running PHP Built-in web server without providing a router script.

From PHP's documentation:
> If a PHP file is given on the command line when the web server is started it is treated as a "router" script. The script is run at the start of each HTTP request. If this script returns FALSE, then the requested resource is returned as-is. Otherwise the script's output is returned to the browser.

When providing a router script every single request will go through it, including asset files. We would need to `return false` in the router script for asset files.
Without it, the webserver will first try to serve the requested file. If the file does not exist, index.php will be served instead.