PHP在下拉列表中显示菜单树

Function to menu tree:

菜单树的功能:

function category_tree($parent_id)
{
    $sql = "SELECT * FROM menu WHERE parent_id ='".$parent_id."'";
    $result = $conn->query($sql);

    echo "<div id=\"wrapper\">
    <div id=\"navmenu\">";
    while($row = mysqli_fetch_object($result)):
        $i = 0;
        if ($i == 0) echo '<ul>';
        echo '<li>' . $row->name;
        category_tree($row->id);
        echo '</li>';
        $i++;
        if ($i > 0) echo '</ul>';
    endwhile;
    echo "</div>
    </div>";
}
function cat