<?php

function post_xml($url, $xml){ 
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_POST,   1);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1 );
    curl_setopt($ch, CURLOPT_POSTFIELDS, $xml);

    $response = curl_exec($ch);
    curl_close($ch);
    return $response;
}  

$xml = new XMLWriter(); 
$xml->openMemory(); 
$xml->startDocument('1.0', 'UTF-8');

$xml->startElement('mds');

    $xml->startElement('auth');
        $xml->writeElement('login', 'login');
        $xml->writeElement('pass', 'hasło');
    $xml->endElement();

    $xml->startElement('request');
        $xml->writeElement('type', 'offers');
        $xml->startElement('conditions');
            $xml->writeElement('par_adt', 2);
        $xml->endElement();

    $xml->endElement();

$xml->endElement();
$xml->endDocument();

$xml_response_string = post_xml('http://mws.merlinx.pl/dataV4/', $xml->outputMemory(true));

if(!$xml_response_string) 
    die('ERROR');

$xml_response = simplexml_load_string($xml_response_string);

?>