php怎么生成6位的不重复的字符串

php生成6位不重复的字符串,用到的工具：notepad++，步骤如下:

1. php代码部分：

echo "刷新输出不同的六个字符组合：".substr(md5(microtime(true)), 0, 6);
?>

说明：此函数先获取当前时间，然后通过md5进行加密，然后再通过substr截取六位数，当前时间是不会重复的，从而保证生成的六个字符串永远不会重复。

效果图：

注意事项：此代码必须在php环境下运行。

给你一个我自己写的function吧。
length：生成的字符串长度，默认是12 
type接收参数三种，默认是all：letter：大小写字母   int：全数字  all：字母加数字
public function len($length=12,$type="all"){
    $int_arr = array("0","1","2","3","4","5","6","7","8","9","0");
    $letter_arr = array("A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z","a","b","c","d","e","f","g","h","i","j","k","l");
    $len_data = array();
    for($i=0;$i<$length;$i++){
        switch($type){
            case "int":
                $rand =  $int_arr;
                break;
            case "letter":
                $rand =  $letter_arr;
                break;
            default:
                $rand =  array_merge($int_arr,$letter_arr);
                break;
        }
        $len_data[] = $rand[rand(0,count($rand)-1)];
    }
    return implode("",$len_data);
}