dataType: "json" won't work

Common issue is that browser prints "something else" before JSON whether that is some readable or unreadable(invisible) char. Try doing something like this:

<?php
//at the very beginning start output buffereing
ob_start();

// do your logic here

// right before outputting the JSON, clear the buffer.
ob_end_clean();

// now print
echo json_encode(array("id" => $realid, "un" => $username, "date" => $date));
?>

Now, all supplement data (before JSON) will be discarded and you should have it working...