<?php
libxml_use_internal_errors(true);           // hide the parsing errors
$dom = new DOMDocument;                 // init new DOMDocument
if($_GET){
    $dom->loadHtmlFile($_GET['open']);
}else{
    $dom->loadHtmlFile('http://www.stackoverflow.com');
}
$dom->loadHtmlFile('http://www.stackoverflow.com'); // load HTML into it
$xpath = new DOMXPath($dom);                // create a new XPath
$nodes = $xpath->query('//a[@href]');       // Find all A elements with a href attribute
foreach($nodes as $node) {               // Iterate over found elements
    $node->setAttribute('href', 'index.php?open=http://www.stackoverflow.com'.$node->getAttribute('href'));     // Change href attribute
}
echo $dom->saveXml();                       // output cleaned HTML
?>