[woojin]

получилось)))
немного не так как хотелось, но:

var data = JSON.stringify({
        iteration: 'start',
        position: '',
        path: '/images/photos/'
    });
xhr_object.open("POST", "core.php", true);
xhr_object.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset=utf-8');
xhr_object.setRequestHeader("X-Requested-With", "XMLHttpRequest");
xhr_object.send('data=' + data);

теперь

$decoded = json_decode($_POST['data'])