1. https://www.facebook.com/photo.php?v=[ID]
  2.  
  3. https://www.facebook.com/photo.php?v=[ID]&type=2&theater
  4.  
  5. $url = 'https://www.facebook.com/photo.php?v=[ID]&type=2&theater';
  6. parse_str( parse_url( $url, PHP_URL_QUERY ), $params );
  7. // $params['v'] will hold the [ID] you need.