require_once("HTTP/Request2.php");

$request = new HTTP_Request2("https://************", HTTP_Request2::METHOD_POST);

$request->setConfig(array(
"ssl_verify_peer" => true,
"ssl_cafile" => "./cacert.pem",
));

cacert.pemは下記からDL
http://curl.haxx.se/docs/caextract.html

http://pear.php.net/manual/ja/package.http.http-request2.config.php