| category_id | category_name | parent_id
| 1           | Electronics   | 0
| 2           | Mobile Phones | 1
| 3           | Computers     | 1
| 4           | Iphone        | 2
| 5           | Samsung Galaxy| 2
| 6           | Asus Laptop   | 3

1. User choose a category from the first selectbox
2. Send ajax to getcategory.php
3. getcategory.php finds out if there is any child category from the user-chosen first category.
4. ajax receive data from getcategory.php
5. if data exist, jQuery create a new select box and put in the fetched data into it.
6. User choose sub-category from the just added selectbox
7. Send ajax to getcategory.php
8. getcategory.php finds out if there is any more child category from the user-chosen sub-category.
9. ajax receive data from getcategory.php
10. if data exist, jQuery create a new select box and put in the fetched data into it.
11. and so on.. until the selectbox have no child category.