Plus2Net: linked drop-down lists (PHP/MySQL)

DEVELOPMENT/LEARNING ONLY. Import sql_dump.txt into a dedicated local test database.
Do not expose editable demo databases or SQL credentials on a public instance.

Files:
- dd3.php and dd3ck.php: three linked lists using PDO and page reloads.
- dd-mysqli.php: separate two-level MySQLi variation. It posts to the same
  display-only dd3ck.php, leaving the third-level selection blank.
- config.php: PDO connection for the three-level demo.
- config-mysqli.php: MySQLi connection for the two-level variation.
- sql_dump.txt: original sample category/subcategory/third-level data.

Set DB_HOST, DB_NAME, DB_USER and DB_PASSWORD environment variables, or edit
config.php and config-mysqli.php for a local test account. Install pdo_mysql
for PDO and mysqli for the MySQLi variant. For PHP 8.x, enable both as needed.

Start a local PHP server, open dd3.php and choose a category. The page reloads
to filter the subcategory, then reloads again for the third level. Submit to
view the selected values; no database records are added or modified.

All original sample records are preserved; the import script now uses MySQL 8-compatible
InnoDB and UTF-8 DDL. Both versions escape
text inserted into HTML, and use prepared statements for user-selected IDs.
Find the related lesson at https://www.plus2net.com/php_tutorial/php_drop_down_list3.php
