<?php
namespace app\common\yun\util;
class StringUtil
{
    public static function round($len = 6)
    {
        $str = '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 M N O P Q R S T U V W X Y Z 0 1 2 3 4 5 6 7 8 9';
        $arr = explode(' ', $str);
        $rand_keys = array_rand($arr, $len);
        shuffle($rand_keys);
        $code = '';
        foreach ($rand_keys as $index=>$key){
            $code .= $arr[$key];
        }
        return $code;
    }
}