if (strstr($_SERVER['REQUEST_URI'],'index.php')) {
header('HTTP/1.0 404 Not Found');
}
Why wont this work? I get a blank page.
Your code is technically correct. If you looked at the headers of that blank page, you'd see a 404 header, and other computers/programs would be able to correctly identify the response as file not found.
Of course, your users are still SOL. Normally, 404s are handled by the web server.
The problem is, once the web server starts processing the PHP page, it's already passed the point where it would handle a 404
In addition to providing a 404 header, PHP is now responsible for outputting the actual 404 page.