function random_code($length = 8,$chars = null){
  if(empty($chars)){
    $chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
  }
  $count = strlen($chars) - 1;
  $code = '';
  while( strlen($code) < $length){
    $code .= substr($chars,rand(0,$count),1);
  }
  return $code;
}
echo random_code;//A1zYbN5X