How to initialize $wpdb?

I have a .php page in the theme root to check the data of one CUSTOM form.
After receiving this data I need to do a query in a custom mysql table, so I need $wpdb, but I can’t use it directly (or doing global $wpdb) because it is a phisical .php file, so the rewrite rule will not affect the request (the request is not passed to index.php because the file exists).

So In this case how can I create an object $wpdb?

Thanks

First of all you probably created an isolated file that is not under the umbrella of WP files. That’s why you are not getting $wpdb. I guess you may not following the general rules/conventions of theme development. my question is now how are you accessing the file?

whatever, if you include the wp_config.php in your file, you will get the $wpdb in your file.

considering in a directory under themes directory, here is how you include/require the file

require_once ('../../../wp-config.php');

you may need to alter the path based on your system.