PayPal Integration Not Working - php

Question

I have PayPal integrated on my website. There's a form on my site which takes the customer to PayPal to finalise the payment then it returns them to my site. My site then sends a request back to PayPal with the transaction token for PDT so that I can run a few checks then auto-credit my customer's with the product they bought.
The system uses PHP and cURL to send process all this.
When I use: "www.sandbox.paypal.com/cgi-bin/webscr"
Along with the sandbox credentials everything works fine, everything.
Once I change it all to "www.paypal.com/cgi-bin/webscr"
It does not work, My code tells me that the request fails.
My Code:
$pp_hostname = "www.paypal.com";
// read the post from PayPal system and add 'cmd'
$req = 'cmd=_notify-synch';
$tx_token = $_GET['tx'];
$auth_token = "#######-hashed_out_here-########";
$req .= "&tx=$tx_token&at=$auth_token";
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://$pp_hostname/cgi-bin/webscr");
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_RETURNTRANSFER,1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $req);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 1);
//set cacert.pem verisign certificate path in curl using 'CURLOPT_CAINFO' field here,
//if your server does not bundled with default verisign certificates.
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 2);
curl_setopt($ch, CURLOPT_HTTPHEADER, array("Host: $pp_hostname"));
$res = curl_exec($ch);
curl_close($ch);
if(!$res){
$this->twig->error("Request failed.");
}else{
$lines = explode("\n", $res);
$keyarray = array();
if (strcmp ($lines[0], "SUCCESS") == 0) {
for ($i='1'; $i<count($lines);$i++){
$test = explode("=", $lines[$i]);
if(empty($test[1])){
$keyarray[urldecode($test[0])] = '';
}else{
$keyarray[urldecode($test[0])] = urldecode($test[1]);
}
}
//give product
} else if (strcmp ($lines[0], "FAIL") == 0) {
$this->twig->error('Transaction failed!');
}
}
Not sure why I'm having a problem so I don't know how to go about fixing.
My PayPal Account is set up as Business, has PDT and IPN turned on as well as Auto-Return
EDIT: Also there isn't an error occurring with the cURL.
OK so if(!($res = curl_exec($ch))) is returning true, i.e its not working