Ho creato il tutto seguendo un tutorial, ecco il codice:

index.html
codice:
<form class="paypal" action="payments.php" method="post" id="paypal_form" target="_blank">    
	<input type="hidden" name="cmd" value="_xclick" /> 
    <input type="hidden" name="no_note" value="1" />
    <input type="hidden" name="lc" value="UK" />
    <input type="hidden" name="currency_code" value="EUR" />
    <input type="hidden" name="bn" value="PP-BuyNowBF:btn_buynow_LG.gif:NonHostedGuest" />
    <input type="hidden" name="first_name" value="Customer's First Name"  />
    <input type="hidden" name="last_name" value="Customer's Last Name"  />
    <input type="hidden" name="payer_email" value="customer@example.com"  />
    <input type="hidden" name="item_number" value="123456" / >
    <input type="submit"  value="Submit Payment"/>
</form>
payments.php
Codice PHP:
// Database variables
$host = "localhost"; //database location
$user = "xxx"; //database username
$pass = "xxx"; //database password
$db_name = "xxx"; //database name

// PayPal settings
$paypal_email = 'paypal@example.com';
$return_url = 'http://gaetanodemitri.altervista.org/pay/payment-successful.htm';
$cancel_url = 'http://gaetanodemitri.altervista.org/pay/payment-cancelled.htm';
$notify_url = 'http://gaetanodemitri.altervista.org/pay/payments.php';

$item_name = 'Nome del prodotto';
$item_amount = 1.00;

// Include Functions
include("functions.php");

//Database Connection
$link = mysql_connect($host, $user, $pass);
mysql_select_db($db_name);


// Check if paypal request or response
if (!isset($_POST["txn_id"]) && !isset($_POST["txn_type"])){

    // Firstly Append paypal account to querystring
    $querystring .= "?business=".urlencode($paypal_email)."&";    
    
    // Append amount& currency (£) to quersytring so it cannot be edited in html
    
    //The item name and amount can be brought in dynamically by querying the $_POST['item_number'] variable.
    $querystring .= "item_name=".urlencode($item_name)."&";
    $querystring .= "amount=".urlencode($item_amount)."&";
    
    //loop for posted values and append to querystring
    foreach($_POST as $key => $value){
        $value = urlencode(stripslashes($value));
        $querystring .= "$key=$value&";
    }
    
    // Append paypal return addresses
    $querystring .= "return=".urlencode(stripslashes($return_url))."&";
    $querystring .= "cancel_return=".urlencode(stripslashes($cancel_url))."&";
    $querystring .= "notify_url=".urlencode($notify_url);
    
    // Append querystring with custom field
    //$querystring .= "&custom=".USERID;
    
    // Redirect to paypal IPN
    header('location:[url]https://www.sandbox.paypal.com/cgi-bin/webscr[/url]'.$querystring);
    exit();

}else{

    // Response from Paypal

    // read the post from PayPal system and add 'cmd'
    $req = 'cmd=_notify-validate';
    foreach ($_POST as $key => $value) {
        $value = urlencode(stripslashes($value));
        $value = preg_replace('/(.*[^%^0^D])(%0A)(.*)/i','${1}%0D%0A${3}',$value);// IPN fix
        $req .= "&$key=$value";
    }
    
    // assign posted variables to local variables
    $data['item_name']            = $_POST['item_name'];
    $data['item_number']         = $_POST['item_number'];
    $data['payment_status']     = $_POST['payment_status'];
    $data['payment_amount']     = $_POST['mc_gross'];
    $data['payment_currency']    = $_POST['mc_currency'];
    $data['txn_id']                = $_POST['txn_id'];
    $data['receiver_email']     = $_POST['receiver_email'];
    $data['payer_email']         = $_POST['payer_email'];
    $data['custom']             = $_POST['custom'];
        
    // post back to PayPal system to validate
    $header = "POST /cgi-bin/webscr HTTP/1.0\r\n";
    $header .= "Content-Type: application/x-www-form-urlencoded\r\n";
    $header .= "Content-Length: " . strlen($req) . "\r\n\r\n";
    
    $fp = fsockopen ('ssl://www.sandbox.paypal.com', 443, $errno, $errstr, 30);    
    
    if (!$fp) {
        // HTTP ERROR
    } else {    
        
        fputs ($fp, $header . $req);
        while (!feof($fp)) {
            $res = fgets ($fp, 1024);
            if (strcmp($res, "VERIFIED") == 0) {
            
                // Used for debugging
                @mail("gaetano.demitri@gmail.com", "PAYPAL DEBUGGING", "Verified Response
data = <pre>".print_r($post, true)."</pre>");
                        
                // Validate payment (Check unique txnid & correct price)
                $valid_txnid = check_txnid($data['txn_id']);
                $valid_price = check_price($data['payment_amount'], $data['item_number']);
                // PAYMENT VALIDATED & VERIFIED!
                if($valid_txnid && $valid_price){        
                        
                    $orderid = updatePayments($data);        
                    if($orderid){    
                        echo "Payment has been made & successfully inserted into the Database";                    
                    }else{                                
                        echo "Error inserting into DB";
                        // E-mail admin or alert user
                    }
                }else{                    
                    // Payment made but data has been changed
                    // E-mail admin or alert user
                }                        
            
            }else if (strcmp ($res, "INVALID") == 0) {
            
                // PAYMENT INVALID & INVESTIGATE MANUALY! 
                // E-mail admin or alert user
                
                // Used for debugging
                @mail("gaetano.demitri@gmail.com", "PAYPAL DEBUGGING", "Invalid Response
data = <pre>".print_r($post, true)."</pre>");
            }        
        }        
    fclose ($fp);
    }    
} 
Funziona tutto perfettamente, però c'è un problema.

Non mi inserisce i record all'interno della tabella "payments" presente nel database "my_gaetanodemitri". Non riesco a capire il perchè.

functions.php
Codice PHP:
// functions.php
function check_txnid($tnxid){
    global $link;
    return true;
    $valid_txnid = true;
    //get result set
    $sql = mysql_query("SELECT * FROM `payments` WHERE txnid = '$tnxid'", $link);        
    if($row = mysql_fetch_array($sql)) {
        $valid_txnid = false;
    }
    return $valid_txnid;
}

function check_price($price, $id){
    $valid_price = false;
    //you could use the below to check whether the correct price has been paid for the product
    
    /* 
    $sql = mysql_query("SELECT amount FROM `products` WHERE id = '$id'");        
    if (mysql_numrows($sql) != 0) {
        while ($row = mysql_fetch_array($sql)) {
            $num = (float)$row['amount'];
            if($num == $price){
                $valid_price = true;
            }
        }
    }
    return $valid_price;
    */
    return true;
}

function updatePayments($data){    
    global $link;
    if(is_array($data)){                
        $sql = mysql_query("INSERT INTO `payments` (txnid, payment_amount, payment_status, itemid, createdtime) VALUES (
                '".$data['txn_id']."' ,
                '".$data['payment_amount']."' ,
                '".$data['payment_status']."' ,
                '".$data['item_number']."' ,
                '".date("Y-m-d H:i:s")."' 
                )", $link);
    return mysql_insert_id($link);
    }
} 
Praticamente la primva volta lui entra nel primo if, presente in payments.php
codice:
if (!isset($_POST["txn_id"]) && !isset($_POST["txn_type"])){
Però dopo non riesce più ad andare nell'else una volta che vengono settate "txn_id" e "txn_type", come posso risolvere questa cosa?