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+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');