如何创建这种唯一的KEY

<?phpfunction getRandStr($len){         $chars = array("0", "1", "2","3", "4", "5", "6", "7", "8", "9"); //换成a-z,A-Z 52种        $charsLen = count($chars) - 1;         shuffle($chars);        $output = "";        for ($i=0; $i<$len; $i++){             $output .= $chars[mt_rand(0, $charsLen)];         }          return $output;      }?>

function getRandStr($len){	$str = "ABCDEFGHIJKLMNOPQRSTUVWXYZzbcdefghijklmnopqrstuvwxyz";	$output = "";	for ($i=0; $i<$len; $i++){ 		$output .= $str{mt_rand(0, 52)};	}  	return $output;  }