WordPress search modification. Read the description!

search-modification.php

<?php
/**
 * Modification of the default WordPress search
 *
 * By default, the WordPress search is very rigid. It checks the post table's
 * title, content and excerpt with a '%LIKE%' keyword. So the search is very
 * limited.
 * This will modify the search clause and do a fulltext search. For more information
 * about fulltext you can read about it here: https://www.digitalocean.com/community/tutorials/how-to-improve-database-searches-with-full-text-search-in-mysql-5-6-on-ubuntu-16-04
 *
 * For it to work, your database has to support it, and you'll need to modify the tables
 * you want to search through. You can do it with these SQL queries:
 *
 * ALTER TABLE wp_posts ADD FULLTEXT (post_title);
 * ALTER TABLE wp_posts ADD FULLTEXT (post_content);
 * ALTER TABLE wp_postmeta ADD FULLTEXT (meta_value);
 *
 * Depending on the content you have in those tables, it may take up to a minute to add
 * FULLTEXT to those columns.
 *
 * What this modification does is that it uses fulltext to create a relevance to the search
 * result, and order by that number.
 *
 * @package  search-modification
 */

add_action( 'pre_get_posts','add_search_variable', 10 );

if ( ! function_exists( 'add_search_variable' ) ) {
  /**
   * Add query variable to search
   *
   * @param WP_Query $wp_query The WP_Query instance (passed by reference).
   * @since 1.0.0
   */
  function add_search_variable( $wp_query ) {
    if ( ! is_admin() && $wp_query->is_main_query() ) {
      if ( $wp_query->is_search ) {
        $wp_query->set( 'query_id', 'distinct_query_id' );
        $wp_query->set( 'post_status', 'publish' );
        $wp_query->set( 'perm', 'readable' );
      }
    }
  }
}

add_filter( 'posts_clauses', 'modify_search_query_clauses', 20, 2 );

if ( ! function_exists( 'modify_search_query_clauses' ) ) {
  /**
   * Filter for the search results query
   *
   * @param  array    $clauses  Query clauses to modify.
   * @param  WP_Query $wp_query The WP_Query instance (passed by reference).
   * @return array              Modified query clauses.
   */
  function modify_search_query_clauses( $clauses, $wp_query ) {
    global $wpdb;
    if ( ! is_admin() && ( is_search() || isset( $wp_query->query_vars['query_id'] ) && $wp_query->query_vars['query_id'] === 'distinct_query_id' ) ) {
      $query_string = esc_html( $wp_query->query_vars['s'] );

      if ( defined( 'ICL_LANGUAGE_CODE' ) ) {
        $current_lang = ICL_LANGUAGE_CODE;
      } else {
        $current_lang = 'hr';
      }

      $clauses['distinct'] = "DISTINCT {$wpdb->posts}.*,";

      $clauses['fields'] = " GREATEST(
                              COALESCE(MATCH ({$wpdb->posts}.post_title) AGAINST ('\"{$query_string}\" @4' IN BOOLEAN MODE), 1) * 100,
                              COALESCE(MATCH ({$wpdb->posts}.post_content) AGAINST ('\"{$query_string}\" @4' IN BOOLEAN MODE), 1),
                              COALESCE(MATCH ({$wpdb->postmeta}.meta_value) AGAINST ('\"{$query_string}\" @4' IN BOOLEAN MODE), 1)
                            ) AS score";

      $clauses['join'] = "LEFT JOIN {$wpdb->postmeta} ON {$wpdb->posts}.ID = {$wpdb->postmeta}.post_id";

      $clauses['where'] = "AND ({$wpdb->posts}.post_type = 'post'
                            OR {$wpdb->posts}.post_type = 'page'
                            OR {$wpdb->posts}.post_type = 'faq-type')
                           AND {$wpdb->posts}.post_type != 'wpcf7_contact_form'
                           AND {$wpdb->posts}.post_type != 'revision'
                           AND (
                             MATCH ({$wpdb->posts}.post_title) AGAINST ('\"{$query_string}\" @4' IN BOOLEAN MODE) > 0 OR
                             MATCH ({$wpdb->posts}.post_content) AGAINST ('\"{$query_string}\" @4' IN BOOLEAN MODE) > 0 OR
                             MATCH ({$wpdb->postmeta}.meta_value) AGAINST ('\"{$query_string}\" @4' IN BOOLEAN MODE) > 0
                           )
                           AND {$wpdb->posts}.post_status = 'publish'";

      $clauses['orderby'] = 'score DESC';
    }

    return $clauses;
  }
}