curl_setopt cacert.pem for paypal - php

Question

I'm trying to set up a callback handler for IPN (paypal) verification. I know what curl does, but I don't know what 'cacert.pem' is (certificate...?).
this is where the callback.php file fails (exists):
curl_setopt($ch, CURLOPT_CAINFO, 'cacert.pem');
if( !($res = curl_exec($ch)) ) {
echo ("Got " . curl_error($ch) . " when processing IPN data");
curl_close($ch);
exit;
}
curl_close($ch);
With the following error message:
Got error setting certificate verify locations: CAfile: cacert.pem
CApath: none when processing IPN data
So I downloaded cacert.pem and copied to the directory where the callback.php file is.
This is a comment before the code above:
// In wamp like environments that do not come bundled with root authority certificates,
// please download 'cacert.pem' from "http://curl.haxx.se/docs/caextract.html" and set the directory path
// of the certificate as shown below.
I'm using XAMPP, this would be the problem?
And finally this is the whole php file directly from developer page:
https://www.x.com/developers/PayPal/documentation-tools/code-sample/216623
<?php
// STEP 1: Read POST data
// reading posted data from directly from $_POST causes serialization
// issues with array data in POST
// reading raw POST data from input stream instead.
$raw_post_data = file_get_contents('php://input');
$raw_post_array = explode('&', $raw_post_data);
$myPost = array();
foreach ($raw_post_array as $keyval) {
$keyval = explode ('=', $keyval);
if (count($keyval) == 2)
$myPost[$keyval[0]] = urldecode($keyval[1]);
}
// read the post from PayPal system and add 'cmd'
$req = 'cmd=_notify-validate';
if(function_exists('get_magic_quotes_gpc')) {
$get_magic_quotes_exists = true;
}
foreach ($myPost as $key => $value) {
if($get_magic_quotes_exists == true && get_magic_quotes_gpc() == 1) {
$value = urlencode(stripslashes($value));
} else {
$value = urlencode($value);
}
$req .= "&$key=$value";
}
// STEP 2: Post IPN data back to paypal to validate
$ch = curl_init('https://www.sandbox.paypal.com/cgi-bin/webscr');
curl_setopt($ch, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_1_1);
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);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 2);
curl_setopt($ch, CURLOPT_FORBID_REUSE, 1);
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Connection: Close'));
// In wamp like environments that do not come bundled with root authority certificates,
// please download 'cacert.pem' from "http://curl.haxx.se/docs/caextract.html" and set the directory path
// of the certificate as shown below.
curl_setopt($ch, CURLOPT_CAINFO, 'cacert.pem');
if( !($res = curl_exec($ch)) ) {
echo ("Got " . curl_error($ch) . " when processing IPN data");
curl_close($ch);
exit;
}
curl_close($ch);
// STEP 3: Inspect IPN validation result and act accordingly
if (strcmp ($res, "VERIFIED") == 0) {
// check whether the payment_status is Completed
// check that txn_id has not been previously processed
// check that receiver_email is your Primary PayPal email
// check that payment_amount/payment_currency are correct
// process payment
// assign posted variables to local variables
$item_name = $_POST['item_name'];
$item_number = $_POST['item_number'];
$payment_status = $_POST['payment_status'];
$payment_amount = $_POST['mc_gross'];
$payment_currency = $_POST['mc_currency'];
$txn_id = $_POST['txn_id'];
$receiver_email = $_POST['receiver_email'];
$payer_email = $_POST['payer_email'];
$string = "------------------------\r\n";
$string .= "item_name: ".$item_name."\r\n";
$string .= "item_number: ".$item_number."\r\n";
$string .= "payment_status: ".$payment_status."\r\n";
$string .= "payment_amount: ".$payment_amount."\r\n";
$string .= "payment_currency: ".$payment_currency."\r\n";
$string .= "txn_id: ".$txn_id."\r\n";
$string .= "receiver_email: ".$receiver_email."\r\n";
$string .= "payer_email: ".$payer_email."\r\n";
$fp = fopen('test.log', 'w');
fwrite($fp, $string."\r\n");
fclose($fp);
} else if (strcmp ($res, "INVALID") == 0) {
// log for manual investigation
}
?>

Answer 1

As per the comment you need to set the (full) directory path, if the pem file is in the same directory as the script then this should work:
curl_setopt($ch, CURLOPT_CAINFO, dirname(__FILE__) . '/cacert.pem');
For an explanation of what the cacert.pem file is, check this accepted answer.