var data = JSON.stringify({
progress: 'start',
position: '',
path: '/images/photos/'
});
var data = 'progress=start&position&path=/images/photos/';
xhr_object.open("POST", "core.php", true);
xhr_object.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
PS. HTTP_RAW_POST_DATA, HTTP_RAW_DATA нет.