Order alphabetically SimpleXML array by attribute

I need a bit of help ordering a SimpleXML array in an alphabetical order using one of it’s attribute ($url). I have the following code that generate a page:

<?php
$xml = simplexml_load_file($xml_url);
$ids = array();

foreach($xml->list as $id) {
   $ids[] = $id;
}

foreach ($ids as $id) {
   PopulatePage($id);
}

function PopulatePage($id) {
   $url = $id->url;
   $img = $id->img;
   //OTHER CODES TO GENERATE THE PAGE
}?>

The problem I have is that, if I convert the SimpleXMLObject to an Array and then order it, the code stops working since $id is no more valid. What can I do? Thanks!