/**
 * Creates recursively a nested HTML UL of array keys.
 * @param array $array Array
 * @return string Nested UL string
 */
function Get_Array_Keys_UL($array=array()) {
 $recursion=__FUNCTION__;
 if (empty($array)) return '';
 $out='<ul>'."\n";
 foreach ($array as $key => $elem)
   $out .= '<li>'.$key.$recursion($elem).'</li>'."\n";
 $out .= '</ul>'."\n"; 
 return $out;
}

$ul=Get_Array_Keys_UL($_POST);
echo $ul;