How to include $wpdb in wordpress plugin?

Question

I\'ve been developing for some time on a plugin in wordpress, but one problem keeps bugging me. I want to export a database-table as an excel file

Answer 1

If you're creating a WordPress plugin, you don't need to include those files manually.

If you want to export your table, why don't you create a function/class for it and pass the $wpdb to it (if you need it). You can also use the normal MySQLi-class (from PHP) do access your MySQL Database.

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If you simply want to access the MySQL Database with the stored login-values which are used by WordPress, you can include the wp_config-file from the WordPress root-directory. It has some (self explaining) global fields which you could use to connect to your database:

include "WP-ROOT-PATH/wp-config.php";
$db = new mysqli(DB_HOST, DB_USER, DB_PASSWORD, DB_NAME);
// Test the connection:
if (mysqli_connect_errno()){
    // Connection Error
    exit("Couldn't connect to the database: ".mysqli_connect_error());
}

After that you have a instance of the MySQLi-class (as mentioned above) which you can use to access your Database.

I'm however not sure if this is the perfect way to go but it sure works.

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To Debug WordPress (if something doesn't work and there is no Error-Message) you should activate debugging in the wp-config.php-file:

/**
 * For developers: WordPress debugging mode.
 *
 * Change this to true to enable the display of notices during development.
 * It is strongly recommended that plugin and theme developers use WP_DEBUG
 * in their development environments.
 */
define('WP_DEBUG', true);

Also, if you're testing your PHP-Scripts on a local server, you should turn the display_error to on in your php.ini-file:

; This directive controls whether or not and where PHP will output errors,
; notices and warnings too. Error output is very useful during development, but
; it could be very dangerous in production environments.
display_errors = On

However this should only be done on your local test-server, not in a productive scenario.