php递归函数无法正常工作

我有一张分类表

id=1，名称=手工制作，父项=0

id=2，名称=工厂制造，父项=0

id=3，name=chairs，parent=1

id=4，name=tables，parent=1

id=5，name=old chairs，parent=3

在菜单上，如果访客点击类别旧椅子，将进入第页

products.php？category_id=5

然后在这个页面上，我需要知道什么是主要的category_id，应该用category_id=1手工制作

所以在这一页我想说

if isset($_REQUEST['category_id']){ 
do the function till find the main parent, 
$mainparentid = main parent category_id 
}else { $mainparentid = ''; 
} echo $mainparentid; 

这是我的PHP代码

if (isset($_REQUEST['category_id'])) {

    function getParent($id) { 
      global $connection;   
      $query_rsCategoryId = "SELECT * FROM categories 
      WHERE category_id = '".$_REQUEST['category_id']."'";
      $rsCategoryId = mysql_query($query_rsCategoryId, $connection);
      $row_rsCategoryId = mysql_fetch_assoc($rsCategoryId);
      $parent = $row_rsCategoryId['category_parent'];
      if (mysql_num_rows($rsCategoryId) < 1) {
        // Error handling, entry with id $id not found
        return null;
      }
      if ($parent == 0) {
        return $id;
      } else {
        return getParent($parent);
      }
    }
$mainparentid = getParent($id);

}else {

    $mainparentid ='none';
}
echo $mainparentid ;

---

您总是从$\请求中输入类别的ID。换行：

WHERE category_id = '".$_REQUEST['category_id']."'";

致：

---

您使用的是来自请求的父id，而不是通过函数参数提供的

  // notice $id instead of $_REQUEST['category_id']
  $query_rsCategoryId = "SELECT * FROM categories WHERE category_id = '{$id}'";

  // notice $id instead of $_REQUEST['category_id']
  $query_rsCategoryId = "SELECT * FROM categories WHERE category_id = '{$id}'";