[destus]

Ну или если очень хочется JSON

xhr_object = create_ajax();
var data = JSON.stringify({
        progress: 'start',
        position: '',
        path: '/images/photos/'
});
xhr_object.open("POST", "core.php", true);
xhr_object.setRequestHeader("Content-Type", "application/json;charset=UTF-8");
xhr_object.send(data);

php

$json = file_get_contents('php://input');
$json_decode = json_decode($json, true);