Estou tentando configurar um manipulador de retorno de chamada para verificação de IPN (paypal). Eu sei o que a onda faz, mas eu não sei o que 'cacert.pem' é (certificado ...?).

é aí que o arquivo callback.php falha (existe):

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);

Com a seguinte mensagem de erro:

Obteve o certificado de configuração de erro para verificar localizações: CAfile: cacert.pem CApath: nenhum ao processar dados IPN

Então baixei cacert.pem e copiei para o diretório onde está o arquivo callback.php.

Este é um comentário antes do código acima:

// 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.

Estou usando o XAMPP, esse seria o problema?

E finalmente este é todo o arquivo php diretamente da página do desenvolvedor: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
}
?>