php如何生成不重复随机字符串

function rand($len)
 {
 $chars='ABCDEFGHIJKLMNOPQRSTUVWXYZ01234567abcdefghijklmnopqrstuvwxyz';
 $string=time();
 for(;$len>=1;$len--)
 {
 $position=rand()%strlen($chars);
 $position2=rand()%strlen($string);
 $string=substr_replace($string,substr($chars,$position,1),$position2,0);
 }
 return $string;
 }

<?php
$numbers = range (1,50);
//shuffle 将数组顺序随即打乱
shuffle ($numbers);
//array_slice 取该数组中的某一段
$num=6;
$result = array_slice($numbers,0,$num);
print_r($result);
?>