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<?php
      $post_id = $post->ID; // current post ID
      $cat = get_the_category(); 
      $current_cat_id = $cat[0]->cat_ID; // current category ID 
      
      $args = array( 
      'category' => $current_cat_id,
      'orderby'  => 'post_date',
      'order'    => 'DESC'
      );
      $posts = get_posts( $args );
      // get IDs of posts retrieved from get_posts
      $ids = array();
      foreach ( $posts as $thepost ) {
      $ids[] = $thepost->ID;
      }
      // get and echo previous and next post in the same category
      $thisindex = array_search( $post_id, $ids );
      $previd    = isset( $ids[ $thisindex - 1 ] ) ? $ids[ $thisindex - 1 ] : 0;
      $nextid    = isset( $ids[ $thisindex + 1 ] ) ? $ids[ $thisindex + 1 ] : 0;
      
      if ( $previd ) {
      ?><a rel="prev" class="btn" href="<?php echo get_permalink($previd) ?>">Précédent</a><?php
      }
      if ( $nextid ) {
      ?><a rel="next" class="btn" href="<?php echo get_permalink($nextid) ?>">Suivant</a><?php
} ?>