The following code makes unified diff of two articles.
<?php
$old_article = file_get_contents('./old_article.txt');
$new_article = $_REQUEST['article']; /* Let's say that someone pasted a new article to html form */
$diff = xdiff_string_diff($old_article, $new_article, 1);
if (is_string($diff)) {
echo "Differences between two articles:\n";
echo $diff;
}
?>