使用贝宝IPN通过电子邮件发送随机字符串

好吧，我切入正题，我有这样的代码，它会产生一个随机字符串：

function random_string()
{      
    $character_set_array = array();
    //$character_set_array[] = array('count' => 2, 'characters' => 'AA');
    $character_set_array[] = array('count' => 8, 'characters' => '0123456789');
    $temp_array = array();
    foreach ($character_set_array as $character_set) {
        for ($i = 0; $i < $character_set['count']; $i++) {
            $temp_array[] = $character_set['characters'][rand(0, strlen($character_set['characters']) - 1)];
        }
    }
    shuffle($temp_array);
    $pinstart = 'AA';
    $pinend = implode('', $temp_array);
    $pin = $pinstart.$pinend;
    echo $pin;
}

然后在我的贝宝IPN我有这个：

if($p->ipn_data["mc_gross"] > 0 && strcmp ($p->ipn_data["business"],$EMAIL) == 0 && ($p->ipn_data["item_number"]) == 1)  {
         $user = $p->ipn_data["custom"];
             $date = $p->ipn_data["payment_date"];
             $prodid = $p->ipn_data["item_number"];
             $amount = $p->ipn_data["mc_gross"];
             $amountTickets = 1;
             $email = 'email@hotmail.com';
             $subject = '[CODE] - Thank you for your donation';
             $message = 'Your CODE IS, <? echo random_string(); ?>';
             mail("$payer_email", "$subject",
             $message.random_string(), "From: $email" );
             $user = str_replace("_", " ", $user);
             $user = str_replace("-", " ", $user);
             $user = mysql_real_escape_string($user);
         }

它发送电子邮件，但它的"你的代码是，它不会发布随机字符串。"

我试过放入$pin，$pin=random_string（）；print$pin nothing work