Get Related Posts Based On The Post Content In WordPress

To get the list of similar posts, based on the current post’s content; you must register this function in your WordPress theme’s functions.php file.

function custom_related_posts() {
     $current_id = get_the_ID();
     $current_content = get_the_content( $current_id );
     $similar = array();
     $scores = array();
     $percentage = 0;
     $output = '';

     function compare_content( $current_content, $comparison_content ) {
          $current_content = trim( strtolower( $current_content ) );
          $comparison_content = trim( strtolower( $comparison_content ) );
          similar_text( $current_content, $comparison_content, $percentage );
          $formated_percents = number_format( $percentage );
          return $formated_percents; 
     }

     $args = array(
          'post_type' => 'post',
          'posts_per_page' => -1
     );
     $loop = new wp_query( $args );
     while( $loop->have_posts() ) {
          $loop->the_post();
          $score = compare_content( $current_content, get_the_content() );
          if ( $score > 55 && get_the_ID() != $current_id ) {
               array_push( $similar, get_the_ID() );
               array_push( $scores, $score );
          }
     }
     wp_reset_query();

     array_multisort( $scores, SORT_NUMERIC, SORT_DESC, $similar );

     if ( ! empty( $similar ) ) {
          $args = array(
               'post_type' => 'post',
               'posts_per_page' => 3,
               'post__in' => $similar,
               'order_by' => 'ID',
               'order' => 'DESC'
          );
          $loop = new wp_query( $args );
          $output = '<h4>Related guides</h4>';
          $output .= '<ul>';
          $i = -1;
          while ( $loop->have_posts() ) {
               $loop->the_post();
               $i++;
               $output .= '<li><a href="' . get_permalink() . '">' . get_the_title() . '</a> <span>' . $scores[$i] . '% related guide</span></li>';
          }
          $output .= '</ul>';
          wp_reset_query();
     }
     return $output;
}

Note that the above function will return a list of matched posts with the match of 55% when called. See the next section on how to match a different percentage of content.

Usage

Simply echo the new custom_related_posts() function in your single-post.php file, located in your WordPress theme’s directory.

<?php echo custom_related_posts(); ?>