function vidCheck($vid) { 
  $url = 'https://img.youtube.com/vi/'.$vid.'/0.jpg?fresh='.time();
  $headers = get_headers($url);
  return (substr($headers[0], 9, 3) == 200) ? true : false;
}

// usage; video id from url: https://www.youtube.com/watch?v=z3PpphdrEmU
// echo vidCheck('z3PpphdrEmU');