/**
 * @param string $type
 * @param $length
 * @return string
 */
function randomString($type="number,upper,lower",$length){
  $valid_type = array('number','upper','lower');
  $case = explode(",",$type);
  $count = count($case);
  //根据交集判断参数是否合法
  if($count !== count(array_intersect($case,$valid_type))){
    return false;
  }
  $lower = "abcdefghijklmnopqrstuvwxyz";
  $upper = strtoupper($lower);
  $number = "0123456789";
  $str_list = "";
  for($i=0;$i<$count;++$i){
    $str_list .= $$case[$i];
  }
  return substr(str_shuffle($str_list),0,$length);
}
echo randomString("number,upper,lower",12);