php如何在选择下拉菜单中显示类别和子类别？

I have a table where i am saving category and product name :

+-----------------------+
|       products        |
+-----------------------+
| id  name     cat_name |
| 1   lily     flower   |
| 2   hibiscus flower   |
| 3   cat      animal   |
| 4   dog      animal   |
+-----------------------+

I want to have a select list with first cat_name and inside that product 'name'

**flower**      
    -lily
    -hibiscus
**animal**  
    -cat
    -dog 

how this can be achieved in select list. please help.

In html body it is the optgroup tag that groups options.

<select>
    <optgroup label="flower">
        <option value="whatever">lily</option>
        <option value="whatever">hibiscus</option>
    </optgroup>
    ...
</select>

Hope you get the idea. Note that these optgroups look pretty poor. Maybe you need some jquery plugin in additon to get what you want.

if you can get array like this

array(
    array(
        'id' => 1
        'name' => 'lily',
        'cat_name' =>'flower'

    ),
    array(
        'id' => 2
        'name' => 'hibiscus',
        'cat_name' =>'flower'

    ),
    .....
    );

then group data by cat

    $groups = array();
    foreach ($results  as $rs) {
      $key = $rs['cat_name'];
      if (!isset($groups[$key])){
        $groups[$key] = array();
      }
      $groups[$key][] = $rs;
    }

then loop $groups , echo html option , like

//this not php code
 loop  groups {
        echo optgroup opentag
        loop groups[cat_name]{
            echo option tag
        }
        echo optgroup close tag
 }