Sending a 404 error in PHP

if (strstr($_SERVER['REQUEST_URI'],'index.php')){
    header('HTTP/1.0 404 Not Found');
    echo "<h1>404 Not Found</h1>";
    echo "The page that you have requested could not be found.";
    exit();
}

if (strstr($_SERVER['REQUEST_URI'],'index.php')){
    header('HTTP/1.0 404 Not Found');
    echo "<h1>404 Not Found</h1>";
    echo "The page that you have requested could not be found.";
    exit();
}

if(strstr($_SERVER['REQUEST_URI'],'index.php')){
  header('HTTP/1.0 404 Not Found');
  readfile('404missing.html');
  exit();
}

if (strstr($_SERVER['REQUEST_URI'],'index.php')){
  header('HTTP/1.0 404 Not Found');
  exit("<h1>404 Not Found</h1>nThe page that you have requested could not be found.");
}

if($_SERVER['PHP_SELF'] == '/index.php'){
   header('HTTP/1.0 404 Not Found');
   echo "<h1>404 Not Found</h1>";
   echo "The page that you have requested could not be found.";
   die;
}