wordpress, match keywords in content to other posts in the same website

function related_content() {
  $categories = get_the_category( $post->ID );
  if ($categories) {
    $category_ids = array();
    foreach($categories as $individual_category) $category_ids[] = $individual_category->term_id;
    $args=array(
      'category__in' => $category_ids,
      'post__not_in' => array($post->ID),
      'posts_per_page'=> 6, // Number of related posts to display.
      'ignore_sticky_posts'=>1
    );
    $my_query = new wp_query( $args );
    if( $my_query->have_posts() ) {
      echo '<div id="related_posts"><h3>Related Posts</h3><ul>';
      while( $my_query->have_posts() ) {
        $my_query->the_post(); ?>
        <li><div class="relatedthumb"><a href="<? the_permalink()?>" rel="book