Sometimes it's necessary to adjust final output. Here's how.
<?php
function callback($buffer) {
// modify buffer here, and then return the updated code
return $buffer;
}
function buffer_start() { ob_start("callback"); }
function buffer_end() { ob_end_flush(); }
add_action('wp_head', 'buffer_start');
add_action('wp_footer', 'buffer_end');