The following add_filter lines of code that will activate shortcode for Posts Comments, Widgets and Excerpts in WordPress, could also be added into your theme functions.php file.

Or if you wish, you can add it on WP_w3all plugin (that allow you to show phpBB posts into WordPress) inside the wp_w3all.php file (NOTE: when plugin update you’ll lost these modifications).

Open wp_w3all.php and search for line:

add_shortcode( 'w3allforumpost', array( 'WP_w3all_phpbb', 'wp_w3all_get_phpbb_post' ) );

immediately after, add the follow line/s, about what/where you want to activate shortcode

Activate shortcode for Posts Comments:

add_filter( 'comment_text', 'do_shortcode' );

Activate shortcode for Widgets:

add_filter( 'widget_text', 'do_shortcode' );

Activate shortcode for Excerpt:

add_filter( 'the_excerpt', 'do_shortcode');

As said above, you could instead add these “activation” code lines for shortcode in Posts Comments, Widgets or Excerpts in WordPress, into the functions.php file of your WordPress theme.