How to modify xml file using PHP

You can use the DOMDocument from PHP.

You load your file and than loop trough the childNodes of the document.

<?php
$dom=new DOMDocument();
$dom->load("file.xml");

$root=$dom->documentElement; // This can differ (I am not sure, it can be only documentElement or documentElement->firstChild or only firstChild)

$nodesToDelete=array();

$markers=$root->getElementsByTagName('marker');

// Loop trough childNodes
foreach ($markers as $marker) {
    $type=$marker->getElementsByTagName('type')->item(0)->textContent;
    $title=$marker->getElementsByTagName('title')->item(0)->textContent;
    $address=$marker->getElementsByTagName('address')->item(0)->textContent;
    $latitude=$marker->getElementsByTagName('latitude')->item(0)->textContent;
    $longitude=$marker->getElementsByTagName('longitude')->item(0)->textContent;

    // Your filters here

    // To remove the marker you just add it to a list of nodes to delete
    $nodesToDelete[]=$marker;
}

// You delete the nodes
foreach ($nodesToDelete as $node) $node->parentNode->removeChild($node);

echo $dom->saveXML();
?>

You can save your output XML like this

$dom->saveXML(); // This will return the XML as a string
$dom->save('file.xml'); // This saves the XML to a file

To do this parsing in JavaScript you should use jQuery (a small, but powerful library).

You can include the library directly from Google Code Repository.

<script type="text/javascript" src="http://jqueryjs.googlecode.com/files/jquery-1.3.2.min.js"></script>

The library is cross-browser and very small. It should be cached in many cases, because some sites use it from Google Code

$(yourXMLStringOrDocument).find("marker").each(function () {
     var marker=$(this);

     var type=marker.find('type').text();
     var title=marker.find('title').text();
     var address=marker.find('address').text();
     var latitude=marker.find('latitude').text();
     var longitude=marker.find('longitude').text();
});