xml属性是什么工作

I have a function that show a result but its not working this the source code

function push($element, $addchild, $attributes = array()) {
    $this->_indent();
    $this->xml .= '<'.$element;
    foreach ($attributes as $key => $value) {
        $this->xml .= ' '.$key.'="'.htmlentities($value).'"';
    }
    $this->xml .= ">
";
    $this->stack[] = $element;
}

The result is <listing_Last_update> in XML

I want the result like this <listing last_update="2014-08-08 14:15:16"> can you help me in this matter.