php随机生成数字字母组合的方法

function getRandomString($len, $chars=null)
{
    if (is_null($chars)){
        $chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    } 
    mt_srand(10000000*(double)microtime());
    for ($i = 0, $str = '', $lc = strlen($chars)-1; $i < $len; $i++){
        $str .= $chars[mt_rand(0, $lc)]; 
    }
    return $str;
}

echo getRandomString(2);

echo chr(mt_rand(65, 90);

echo chr(mt_rand(97, 122));