page navigation periodical

function mag_check_post_navigation ( $output, $format, $link, $post ) {

// if this post is not the current issue then return blank
if ( !has_term( mag_get_current_issue() , 'issue' , $post) ) return '';
return $output;
}

// add the check to the next and previous post links - make sure 4 parameters are passed so we get $post
add_filter( 'next_post_link' , 'mag_check_post_navigation', 10, 4);
add_filter( 'previous_post_link' , 'mag_check_post_navigation', 10, 4);