McDonald/Stay In Category

WordPress Stay in Category

Author: Mac McDonald
Contact: Use the Contact Us form under About on BlueGrassMiataClub =dot= com

By default, the previous/next_post_link in single.php does not stay in
category. If you view a category archive, then select a single post,
the prev/next link may not show a post in the same category.

If you set the 'in_same_cat' parameter to the previous/next_post_link
calls in single.php to 'true', single.php will stay in category
provided the posts belong only to one category. If posts belong to
more than one category, the prev/next links may point to posts in one
of the categories other than the one from the category
archive. Changing 'in_same_cat' will also affect how a blog works,
because you normally don't want to stay in category when coming from a
blog post list.

Solving this problem requires several steps:

1. Modify archive.php (or category.php, if it exists) to pass the
  'stayincat' parameter in the URL of single posts.

2. Modify single.php to detect the 'stayincat' parameter, set a global
  category id for the use by following filters, and set the
  'in_same_cat' to 'true' in previous/next_post_link.

3. Add filters for previous/next_post_join so that only the global
  category id is selected, instead of all categories for the current
  post.

4. Add filters for previous/next_post_link to pass the 'stayincat'
  parameter in the links to other single pages.

5. Add a replacement function for add_query_arg because it does not
  work on the links in the previous/next_post_link filters.

Here is the code for archive.php (or category.php):

   <?php while (have_posts()) : the_post(); ?>
      <div <?php post_class() ?>>
      <?php $permalink = get_permalink();   // MAM modification to stay in category - see single.php
           if (is_category()) $permalink = add_query_arg('stayincat',get_query_var('cat'),$permalink);  ?>
          <h3 id="post-<?php the_ID(); ?>"><a href="<?php echo $permalink; ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></h3>

Here is the code for single.php:

<div class="navigation">
   <?php $mam_global_stay_in_cat = ($_GET['stayincat']) ? $_GET['stayincat'] : false;
   if ( $mam_global_stay_in_cat ) { ?>
      <div class="alignleft"><?php previous_post_link('&laquo; %link','%title',true) ?></div>
      <div class="alignright"><?php next_post_link('%link &raquo;','%title',true) ?></div>
   <?php } else { ?>
       <div class="alignleft"><?php previous_post_link('&laquo; %link') ?></div>
       <div class="alignright"><?php next_post_link('%link &raquo;') ?></div>
   <?php } ?>
</div>

And here are the filters and functions. They can go in functions.php,
or at the top of single.php:

// MAM - modification to stay in the category set in archive.php
function mam_get_previous_post_join_filter ( $join, $in_same_cat = false, $excluded_categories = '' ) {
   global $mam_global_stay_in_cat;
   if ($mam_global_stay_in_cat) {
     $join = preg_replace('/tt.term_id IN \([^(]+\)/',"tt.term_id IN ($mam_global_stay_in_cat)",$join);
   }
   return $join;
}
function mam_get_next_post_join_filter ( $join, $in_same_cat = false, $excluded_categories = '' ) {
   global $mam_global_stay_in_cat;
   if ($mam_global_stay_in_cat) {
     $join = preg_replace('/tt.term_id IN \([^(]+\)/',"tt.term_id IN ($mam_global_stay_in_cat)",$join);
   }
   return $join;
}
function mam_previous_post_link_filter ($link='') {
  global $mam_global_stay_in_cat;
  //echo '<p>PREV LINK BEFORE:' . htmlspecialchars($link) . '</p>';
  if ($mam_global_stay_in_cat && $link) {
    $link = mam_add_query_arg('stayincat',$mam_global_stay_in_cat,$link);
    //echo '<p>PREV LINK AFTER:' . htmlspecialchars($link) . '</p>';
  }
  return $link;
}
function mam_next_post_link_filter ($link='') {
  global $mam_global_stay_in_cat;
  //echo '<p>NEXT LINK BEFORE:' . htmlspecialchars($link) . '</p>';
  if ($mam_global_stay_in_cat &#038;& $link) {
    $link = mam_add_query_arg('stayincat',$mam_global_stay_in_cat,$link);
    //echo '<p>NEXT LINK AFTER:' . htmlspecialchars($link) . '</p>';
  }
  return $link;
}
 function mam_add_query_arg ($key,$value,$link) {
   // Adds the parameter $key=$value to $link, or replaces it if already there.
   // Necessary because add_query_arg fails on previous/next_post_link.
   if (strpos($link,'href')) {
     $hrefpat = '/(href *= *([\"\']?)([^\"\' ]+)\2)/';
   } else {
     $hrefpat = '/(([\"\']?)(http([^\"\' ]+))\2)/';
   }
   if (preg_match($hrefpat,$link,$matches)) {
      $url = $matches[3];
      $newurl = add_query_arg($key,$value,$url);
      // echo '<p>OLDURL:' . htmlspecialchars($url) . '</p>';
      // echo '<p>NEWURL:' . htmlspecialchars($newurl) . '</p>';
      $link = str_replace($url,$newurl,$link);
   }

   return $link;
}

add_filter('get_previous_post_join','mam_get_previous_post_join_filter');
add_filter('get_next_post_join','mam_get_next_post_join_filter');
add_filter('previous_post_link','mam_previous_post_link_filter');
add_filter('next_post_link','mam_next_post_link_filter');