响应式设计中的PHP动态导航问题

Question

我正在使用PHP，Mysql和bootstrap制作动态菜单。问题在于它正在向每个子元素添加下拉类：

这是我的动态菜单的输出

<ul class="nav navbar-nav">
<li class="open"><a href="#" class="has-submenu highlighted">DEMO<span 
class="sub-arrow">...</span></a>
<ul ''="" class="sm-nowrap" style="display: block; width: auto; top: auto; 
left: 0px; margin-left: 0px; margin-top: 0px; z-index: 3; min-width: 200px; 
max-width: 400px;">
<li><a href="DEMO.php?id=20&amp;page=1" class="has-submenu">DEMO<span 
class="sub-arrow">...</span></a>
<ul 'class="dropdown-menu' "></ul>
 </li>
<li><a href="DEMO.php?id=22&amp;page=1 " class="has-submenu ">DEMO<span 
class="sub-arrow ">...</span></a>
<ul ''=" "></ul>
</li>
<li><a href="DEMO.php?id=19&amp;page=1 " class="has-submenu ">DEMO<span 
class="sub-arrow ">...</span></a>
<ul ''=" ">
</ul>
</li>
<li><a href="DEMO.php?id=25&amp;page=1 " class="has-submenu ">DEMO<span 
class="sub-arrow ">...</span></a>
<ul ''=" ">
</ul>
</li>
</ul>
</li>
</ul>

当我使用引导程序时，我想要的结果类型是

引导导航

<li class="dropdown">
    <a class="dropdown-toggle" data-toggle="dropdown" href="#">Page 1
    <span class="caret"></span></a>
    <ul class="dropdown-menu">
      <li><a href="#">Page 1-1</a></li>
      <li><a href="#">Page 1-2</a></li>
      <li><a href="#">Page 1-3</a></li>
    </ul>
  </li>
</li>

这是我的PHP代码

 function get_menu_tree($parent_id) 
{
global $con;
$menu = "";
$sqlquery = " SELECT * FROM tbl_menu where parent='" .$parent_id . "' order by sort";
$res=mysqli_query($con,$sqlquery);
    $a="";
    while($row=mysqli_fetch_array($res,MYSQLI_ASSOC)) 
{

           if($a==$row['parent']){$cl=null;}else{ $cl="class=dropdown-menu"; 
   }$a=$row['parent'];
       $menu .="<li ><a href='".$row['link']."'>".$row['label']."</a>";

       $menu .= "<ul '".$cl."'>".get_menu_tree($row['id'])."</ul>"; //call  recursively

       $menu .= "</li>";

}

return $menu;
}