Paypal IPN response $res is empty - php

Question

this is the first time I am using Paypal to process a payment. I have set up a developer account, created some test merchant/buyer accounts and successfully created a cart that I sent to paypal sandbox. Here, using one of my buyers accounts, I can, again, successfully complete the purchase. In the account history of paypal, it shows that the transaction was completed. All good till now.
The problem lays in the return url, where I get the IPN. Here is the code I copied from paypal website.
<?php
include($_SERVER['WROOT'].'core/init.php');
//Put together postback info
$postback = 'cmd=_notify-validate';
foreach($_POST as $key =>$value){
$postback .= "&$key=$value";
}
// EMAIL $postback
// build the header string to post back to PayPal system to validate
$header = "POST /cgi-bin/webscr HTTP/1.1\r\n";
$header .= "Host: www.sandbox.paypal.com\r\n";
$header .= "Content-Type: application/x-www-form-urlencoded\r\n";
$header .= "Content-Length: " . strlen($postback) . "\r\n\r\n";
// also tried with $fp = fsockopen ('ssl://www.sandbox.paypal.com', 443, $errno, $errstr, 30);
$fp = fsockopen ('www.sandbox.paypal.com', 80, $errno, $errstr, 30);
if(!$fp){ //no conn
die();
}
//post data back
fputs($fp, $header . $postback);
while(!feof($fp)){
$res=stream_get_contents($fp, 1024);
if((strcmp($res, "VERIFIED")) == 0){
// EMAIL with a success notification
// update the database - THIS IS ONLY A SMALL TEST TO SEE IF THE TRANSACTION IS SUCCESSFUL -
$new = $dbh->prepare(" ISNERT INTO orders(txn_id) VALUES(:txn_id) ");
$new->execute(array( 'txn_id' => $_POST['txn_id'] ));
} else if ( strcmp ($res, "INVALID") == 0 ) {
// EMAIL with a error notification
// LOG THE ERROR TO A FILE
}
}
?>
Ok, first of all I don't do any check to see if the email, gross amount and other parameters are valid, that is something I will do after I can solve this problem.
Anyway, after a buyer pays, my database should update, but it does not.
What I did ?
First of all, I email me the $postback variable as soon as it's created and it worked. I got the email with a huge string of response and all the data was correct.
But the $res variable is not either VERIFIED or INVALID so anything past the fsockopen() does not work.
As I said, the payment on the paypal website is successfull. The documentation is fairly poor and I can't get an answer.
The one thing I want to add is that my website does NOT have a SSL certificate, but I do not store any of the customer data, everything is processed on the secure paypal website. Do I need a SSL certificate ?
One last thing. I tried using this class https://github.com/Quixotix/PHP-PayPal-IPN and the error log message is Invalid response status: 302