Paypal IPN setup

经过几个小时的尝试修复paypal IPN我回到了开始。这非常令人沮丧，但我无法理解这一点。我有一个形式，并将其发送到贝宝，但我不知道如何进一步的后续。

echo '<form method="post" action="'; 
include("settings.php");
if($_POST['payment_buy_ads'] == 'PayPal' && $paypal_status == 1){
echo 'https://www.sandbox.paypal.com/cgi-bin/webscr';
}
if($_POST['payment_buy_ads'] == 'Payza' && $payza_status == 1){
 echo '';
}
if($_POST['payment_buy_ads'] == 'Perfect_Money' && $perfect_money_status == 1){
echo '';
}
echo '">';
echo '

<h3>Website URL</h3>
<input type="url" class="form-control" id="user_url" name="url" placeholder="Whats the URL of the website?" required />
<br>
<h3>Ad Pack Type</h3>
<p class="col-lg-12 ">Chose how many ad credits do you like to buy?</p>
<br>
<br> 
<select name="amount" class="form-control">
<option value="'.$cost_p_1.'">1000 hits - $'.$cost_p_1.'</option><option value="'.$cost_p_2.'">5000 hits - $'.$cost_p_2.'</option><option value="'.$cost_p_3.'">20000 hits - $'.$cost_p_3.'</option><option value="'.$cost_p_4.'">100000 hits - $'.$cost_p_4.'</option><option value="'.$cost_p_5.'">1000000 hits - $'.$cost_p_5.'</option></select><br><br>
<input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="business" value="'.$paypal_email.'">
<input type="hidden" name="item_name" value="Full Page Link Advertising">
<input type="hidden" name="currency_code" value="USD">
<INPUT TYPE="hidden" name="user_email" value="'.$_SESSION['email'].'">
<INPUT TYPE="hidden" name="notify_url" value="'.$site_link.'/ipn.php">
<input type="submit" name="submit" class="btn btn-success btn-lg" value="Create New Ad">
<br>
<br>
'; include("settings.php");
echo '
<br>

<br>
<br>
</form>

这可以完美地到达paypal结帐，但是我如何检查付款是否成功以及用户选择了哪个包。

请一步一步地解释，尽量保持简单。

问候,奥利弗