How to Display a Block on a Page with a Variable URL I want to show a block only on these user pages: .../user/USER_ID/recurring-fees. What will be the code to be placed in the PHP box to make this work?
I'm not sure what you mean by the PHP box.  You could do this with the block's visibility settings.  For example, if you wanted to show the "Powered by Drupal"
 block on only the URLs you mentioned, you would go to...

/admin/structure/block/manage/system/powered-by/configure


...and in the Pages tab of the Visibility Settings, enter...

user/*/recurring-fees


Also make sure you have ticked the radio button for "Only the listed pages".

If you are coding the block yourself, you could do something toward the top of your block function like...

if ('user' !== arg(0) || !is_numeric(arg(1)) || 'recurring-fees' !== arg(2)) {
 return array();
}


See http://api.drupal.org/api/drupal/includes!bootstrap.inc/function/arg/7

But the usual approach is to use the visibility settings.