Before you want to send files or structured data over and AJAX call from PHP you might need to clean all output buffers (ob_start(...)) to be able to trash the junk in the buffers. It especially can happen in CMS like Joomla, WordPress etc.. when other plugin does strange things.

PHP - Clear and close all non-lethal output buffers ob_end_clean_all

<?php

function ob_end_clean_all() {
    $handlers = ob_list_handlers();
    while (count($handlers) > 0 && $handlers[count($handlers) - 1] != 'ob_gzhandler' && $handlers[count($handlers) - 1] != 'zlib output compression') {
        ob_end_clean();
        $handlers = ob_list_handlers();
    }
}