The custom file

/wp-content/plugins/wp-w3all-phpbb-integration/common/custom_functions.php

can be used to fire hooks, declare functions or vars of any kind. Any other plugin can so work along with the integration plugin in any possible aspect as more like.

If the file custom_functions.php exist into the custom created folder named wp-w3all-custom it will be parsed as part of the wp_w3all.php file (where it is included).

To use it, create the folder

/wp-content/plugins/wp-w3all-custom/

copy wp-content/plugins/wp-w3all-phpbb-integration/common/custom_functions.php

and paste it into the /wp-content/plugins/wp-w3all-custom/ created folder.

so you’ll have

/wp-content/plugins/wp-w3all-custom/custom_functions.php

Open it with a text editor: it already contain two simple examples, and can be used for any scope.