Use curl and regex :
myserver.tld/test.php
<?php
$target_url = 'http://demo.whmcs.com/knowledgebase.php';
$this_page = 'test.php';
$tail = ( !empty( $_SERVER['QUERY_STRING'] ) ? '?'.$_SERVER['QUERY_STRING'] : '' );
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $target_url.$tail );
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$output = curl_exec($ch);
curl_close($ch);
// cats
$output = preg_replace( '~^.+?(<h2>categories</h2>.+?</table>).+?$~is', '$1', $output );
// articles
$output = preg_replace( '~^.+?(<h2>articles</h2>.+?)<br /><p align="center">.+?$~is', '$1', $output );
// article
$output = preg_replace( '~^.+?(<h2>.+?)<br /><p align="center">.+?$~is', '$1', $output );
echo preg_replace( '~knowledgebase\.php~i', $this_page, $output );
?>
Note : You'll also need to handle $_POST if you want to enable voting and stuff
¥