htmldiff is a PHP library that displays the difference between two HTML snippets. It is taken from the MediaWiki Visual Diff module and has been modified to be usable outside of MediaWiki.

require("htmldiff/html_diff.php");
$html1 = "<p>This is an example text with a <strong>bold part</strong>.</p>";
$html2 = "<p>This is a changed example text without a bold part.</p>";
echo html_diff($html1, $html2);

Output:

<p>This is <span class="diff-html-removed" id="removed-htmldiff-0">an </span><span class="diff-html-added" id="added-htmldiff-0">a changed </span>example text <span class="diff-html-removed" id="removed-htmldiff-1">with </span><span class="diff-html-added" id="added-htmldiff-1">without </span>a bold part.</p>

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. or see https://www.gnu.org/

These are the original MediaWiki source files that this library is based on: HTMLDiff.php, Diff.php, Nodes.php, Sanitizer.php, Xml.php