header跳轉到一個文件鏈接，如果這個文件鏈接是404，那麼修改文件的後綴，再次重試

$file_url = $_GET['src'];
$file_extensions = array('png', 'jpeg', 'jpg');

foreach ($file_extensions as $extension) {
  $r = pathinfo($file_url, PATHINFO_EXTENSION);
  $new_file_url = str_replace($r, $extension, $file_url);

  if (@get_headers($new_file_url)[0] === 'HTTP/1.1 200 OK') {
    header("Location: $new_file_url");
    exit;
  }
}

header($_SERVER["SERVER_PROTOCOL"] . " 404 Not Found", true, 404);
echo "File not found.";