If created your DOMDocument object using loadHTML() (where the source is from another site) and want to pass your changes back to the browser you should make sure the HTTP Content-Type header matches your meta content-type tags value because modern browsers seem to ignore the meta tag and trust just the HTTP header. For example if you're reading an ISO-8859-1 document and your web server is claiming UTF-8 you need to correct it using the header() function.
<?php
header('Content-Type: text/html; charset=iso-8859-1');
?>