How to convert php array to xml

/**
 * XML编码
 * @param mixed $data 数据
 * @param string $encoding 数据编码
 * @param string $root 根节点名
 * @return string
 */
function xml_encode($data, $encoding='utf-8', $root='think') {
    $xml    = &#39;<?xml version="1.0" encoding="&#39; . $encoding . &#39;"?>&#39;;
    $xml   .= &#39;<&#39; . $root . &#39;>&#39;;
    $xml   .= data_to_xml($data);
    $xml   .= &#39;</&#39; . $root . &#39;>&#39;;
    return $xml;
}
/**
 * 数据XML编码
 * @param mixed $data 数据
 * @return string
 */
function data_to_xml($data) {
    $xml = &#39;&#39;;
    foreach ($data as $key => $val) {
        is_numeric($key) && $key = "item id=\"$key\"";
        $xml    .=  "<$key>";
        $xml    .=  ( is_array($val) || is_object($val)) ? data_to_xml($val) : $val;
        list($key, ) = explode(&#39; &#39;, $key);
        $xml    .=  "</$key>";
    }
    return $xml;
}