Yes, use getJSONP. It's the only way to make cross domain/server async calls. (*Or it will be in the near future). Something like
$.getJSON('your-api-url/validate.php?'+$(this).serialize+'callback=?', function(data){
if(data)console.log(data);
});
The callback parameter will be filled in automatically by the browser, so don't worry.
On the server side ('validate.php') you would have something like this
<?php
if(isset($_GET))
{
//if condition is met
echo $_GET['callback'] . '(' . "{'message' : 'success', 'userID':'69', 'serial' : 'XYZ99UAUGDVD&orwhatever'}". ')';
}
else echo json_encode(array('error'=>'failed'));
?>