Verifying Amazon SNS Notification Signatures

gistfile1.php

<?php

//  get the raw HTTP post data
$postdata = file_get_contents('php://input');

//  the post data is JSON, so let's decode it and grab the various fields
$json = json_decode($postdata);
$subject = $json->Subject;
$message = $json->Message;
$signature = $json->Signature;
$message_id = $json->MessageId;
$timestamp = $json->Timestamp;
$topic_arn = $json->TopicArn;
$type = $json->Type;

//  decode the signature
$decoded_signature = base64_decode($signature);

//  generate the canonical string that will be used to verify the signature
$data = "Message\n$message\nMessageId\n$message_id\n";

//  only add the subject if it is set
if($subject != null) {
    $data .= "Subject\n$subject\n";
}

$data .= "Timestamp\n$timestamp\nTopicArn\n$topic_arn\nType\n$type\n";
$data = utf8_encode($data);

//  grab the Amazon SNS certificate file
$cert = file_get_contents("sns.pem"); 

//  retrieve the public key from the certificate
$pkeyid = openssl_get_publickey($cert) or die("Couldn't read public key");

//  verifiy the canonical string using the public key and the decoded signature
$ok = openssl_verify($data, $decoded_signature, $pkeyid, OPENSSL_ALGO_SHA1);

if ($ok == 1) {
    //  signature was good
} elseif ($ok == 0) {
    //  signature was bad
} else {
    //  and error occurred
}

// free the key from memory
openssl_free_key($pkeyid);

?>

how we get file_get_contents("sns.pem"); this sns.pem file .i know that we have to read SigningCERTURL but how can i get sns.pem ?????