Php 注意：未定义的属性：PDOStatement:：$mysqli_num_中的行

下面是返回以下错误的PHP代码

public static function categoryParentChildTree($parent = 14, $spacing = '', $category_tree_array = '') {

    $db=Db::getConnection();

    if (!is_array($category_tree_array))
        $category_tree_array = array();

    $sql = 'SELECT id,name,parentid FROM category WHERE parentid = 14 ORDER BY id ASC';
    $resCategory=$db->query($sql);

    *if ($resCategory->num_rows > 0) {*
        while($rowCategories = $resCategory->fetch_assoc()) {
            $category_tree_array[] = array("id" => $rowCategories['id'], "name" => $spacing . $rowCategories['name']);
            $category_tree_array = Self::categoryParentChildTree($rowCategories['id'], '    '.$spacing . '- ', $category_tree_array);
        }
    }

    return $category_tree_array;
}

请帮助我这个函数输出这个错误

注意：未定义的属性：PDOStatement:：$mysqli_num_中的行

---

出现您的通知是因为未创建对象

替换行

$resCategory=$db->query（$sql）通过以下方式：

if($resCategory = $db->query($sql)) {
    //continue with your code...
    if ($resCategory->num_rows > 0) {
    ...
}

else {
    // SQL Query didn't return anything
}

$sql='从parentid=''的类别中选择id、名称、parentid。$parent.'ORDER BY id ASC'；if（$resCategory=$db->query（$sql））{//继续您的代码……if（$resCategory->num_rows>0）{