To prevent a parser error when the comment string would contain the character sequence "--", do this:
<?php
$CommentString = 'This contains -- some weird -- characters.';
$CommentNode = $DomDocument->createComment(
str_replace('--', '-'.chr(194).chr(173).'-', $CommentString)
);
?>
This will insert a Soft Hyphen in between the two hyphens which will not cause the parser to error out.