$dom = new DOMDocument('1.0', 'UTF-8');
$dom->loadXML($xml_post);
$msgids = $dom->getElementsByTagName('custom-attribute');
foreach($msgids as $k=>$v){
    $tmp_msg = array();
    $tmp_msg['attr_value'] = $v->getAttribute('attribute-id');
    $tmp_msg['tag_value'] = $v->item(0)->nodeValue;
    $msgid[] = $tmp_msg;
}