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');

?>

When using the XmlReader to read local XML files, remember it the open function requests a URI. Add 'file://' to the front of the FULL path to the XML. Otherwise you may get:

PHP Warning:  XMLReader::open(): Unable to open source data in ...

XML can optionally declare its own encoding:

    <?xml version="1.0" encoding="UTF-8"?>

You can use the $encoding parameter to provide this information (if missing) or override it (if wrong).

Output is always UTF-8 (that's how libxml works).

Windows users remember to enable php_openssl.dll in your php.ini if you want to be able to use this function (and others) with a HTTPS URL.

For some reasons, the open() method keep throwing me this error :

PHP Warning:  XMLReader::open(): Unable to open source data in /var/www/nota/ethamap/fat_xml.php

It doesn't make sense as the xml file target hosted on my server is perfectly reachable. Adding this line before invoking open() fixed it : 

libxml_disable_entity_loader(false); 

Please view https://bugs.php.net/bug.php?id=62577
It is somehow related.