Hello, summer has start here, so sorry for the delay … 2 month of sunshine you know ??
wp_w3all grab credentials before any action hook: on next can be set as option to be activated or not.
Look that wp_w3all, when an user login on WP side, send related phpBB cookies browser. When an user login on phpBB side and come into WP side, than wp_w3all detect and validate phpBB cookie, and login the user into WP.
Now, if you want your custom login code work with wp_w3all, you should take care that your code, send also required phpBB cookies onlogin OR, wp_w3all, will logout you, as can’t find out a valid phpBB cookie to check against. Further more wp_w3all run hooks mainly on init, and not on after_setup_theme: so wp_w3all is executed before your code.
And the latest addition, about external logins even before just grabbing the $_POST array (look addons/ext_plugins_fixes.php) that is included (if you would like to exclude) on file wp_w3all.php
//// workaround for some plugin that substitute wp-login.php default login page
//// ... but that DO NOT reset $_POST array (like some frontend ajax login widget plugin instead do)
require_once( WPW3ALL_PLUGIN_DIR . 'addons/ext_plugins_fixes.php' ); // code, actions and functions
//// END workaround for some plugin that substitute wp-login.php default login page
There are several, really many answer i could give you, hope this will help, if not, just repost.