Sending the right HTTP protocol for headers from PHP is easy, you just need to find which HTTP protocol your server is using. Most servers will be using HTTP/1.1, but allowing PHP to serve the protocol version for you means safer headers for older systems and greater cross-platform compatibility.
Rather than sending
header( 'HTTP/1.1 404 Not Found' );
we're going to send
header( $_SERVER['SERVER_PROTOCOL'] . ' 404 Not Found' );
gives us the HTTP protocol which matches the previous 'HTTP/x.x' format.
This can be used with any header that requires the HTTP protocol to be sent, it is not limited to 404 errors in particular.
Available from PHP 5.4.0, function 'http_response_code' is the alternative to this and sends the correct protocol along with the correct status code description.
Output: HTTP/1.1 404 Not Found