rainborick - 1:55 am on Aug 6, 2012 (gmt 0)
OK, if you're going to persist with the call to header('HTTP/1.0...'), try:
header("HTTP/1.0 404 Not Found", TRUE);
Adding the 'replace' parameter set to TRUE may/should replace any previous instance of that header that would ordinarily be sent by PHP.
On the whole, though, I'd suggest you use header("Location:") as I posted. It's working code I took from one of my sites. It sends the 404 response code and shows the error page to the user. If it doesn't work, check the server error_log to see if there are any errors or warnings from PHP.