How to include php code in a XML file

How could I inlcude the following php code in a xml file or how I change the xml file to a php file?

<?php
foreach ($verbs["a"] as $key => $list) {
echo '<link>'."\n";
echo '<title>'.($verbs["a"][$key]).'</title>'."\n";
echo '<url>a/'.letter($verbs["a"][$key]).'/</url>'."\n";
echo '</link>'."\n";
} 
?>