//提取所有代碼
$pattern = '/<div class="nav" monkey="nav">(.+?)<div class="head-ad">/is';
preg_match($pattern, $string, $match);
//$match[0] 即為<div class="nav" monkey="nav">和<div class="head-ad">之間的所有源碼
echo $match[0];
//然後再提取<li></li>之間的內容
$pattern = '/<li.*?>(.+?)<\/li>/is';
preg_match_all($pattern, $match[0], $results);
$new_arr=array_unique($results[0]);
foreach($new_arr as $kkk){
echo $kkk;
}