Pass additional parameter in the URL

There is a wordpress function for adding $_GET-Params to an URL: add_query_arg(). It works like this:

$link = add_query_arg( array('pagenum' => $your_page_number) );

By default it adds the param to the current page (gets content from $_SERVER[REQUEST_URI]) but you can also pass an optional param to the function if you want the link to go to another page:

$link = add_query_arg( array('pagenum' => $your_page_number), $your_page_link);