<?php
$xmlstr = <<<XML
<?xml version='1.0' standalone='yes'?>
<media:group>
        <media:content url="http://www.url.com" bitrate="450" width="320" duration="32" />
        <media:content url="http://www.url.com" bitrate="212" width="320" duration="32" />
</media:group>
XML;

$data = new SimpleXMLElement($xmlstr);
$result = array();

foreach ($data->content as $entry){
    $subResult = array();
    $attribute = $entry->attributes();
    foreach ($attribute as $key => $value){
        $subResult[(string)$key] = (string)$value;
    }
  $result[] = $subResult;
}

var_dump($result);

/*
user@host ~ $ php test.php
array(2) {
  [0]=>
  array(4) {
    ["url"]=>
    string(18) "http://www.url.com"
    ["bitrate"]=>
    string(3) "450"
    ["width"]=>
    string(3) "320"
    ["duration"]=>
    string(2) "32"
  }
  [1]=>
  array(4) {
    ["url"]=>
    string(18) "http://www.url.com"
    ["bitrate"]=>
    string(3) "212"
    ["width"]=>
    string(3) "320"
    ["duration"]=>
    string(2) "32"
  }
}

*/