/**
 * 生成随机字符串
 * @param $len
 * @return int|string|string[]
 */
function make_str($len)
{
    $chars='ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789abcdefghijklmnopqrstuvwxyz';
    $string=time();
    for(;$len>=1;$len--)
    {
        $position=rand()%strlen($chars);
        $position2=rand()%strlen($string);
        $string=substr_replace($string,substr($chars,$position,1),$position2,0);
    }
    return $string;
}