If you like to read the XML from HTTP whit a POST request, you can use libxml_set_streams_context.
Example:

<?php

$param = array('http' => array(
    'method' => 'POST',
    'header' => "Content-type: application/x-www-form-urlencoded\r\n",
    'content' => http_build_query(array(
        'post_param1' => 'value1',
        'post_param2' => 'value2',
    )),
));
libxml_set_streams_context(stream_context_create($param));
$reader = XMLReader::open('https://example.com/get.php?get_param=value3');

?>