Search Synonyms

// Groups of words that mean the same thing on your site; a search for any of them finds them all. Empty = off. Examples:
// array( 'sofa', 'couch', 'settee' ),
// array( 'tv', 'television' ),
$wpide_search_synonyms = array();

add_filter( 'posts_search', function ( $search, $query ) use ( $wpide_search_synonyms ) {
    global $wpdb;

    if ( '' === $search || ! $wpide_search_synonyms || is_admin() || ! $query->is_main_query() || ! $query->is_search() ) {
        return $search;
    }

    $terms = (array) $query->get( 'search_terms' );

    // Leave exclusions ("-word") and exact-phrase searches to WordPress.
    if ( ! $terms || $query->get( 'exact' ) || $query->get( 'sentence' ) || preg_grep( '/^-/', $terms ) ) {
        return $search;
    }

    $lookup = array();

    foreach ( $wpide_search_synonyms as $group ) {
        $group = array_map( 'strtolower', array_map( 'trim', (array) $group ) );

        foreach ( $group as $word ) {
            $lookup[ $word ] = $group;
        }
    }

    $changed = false;
    $parts   = array();

    foreach ( $terms as $term ) {
        $words = isset( $lookup[ strtolower( $term ) ] ) ? $lookup[ strtolower( $term ) ] : array( $term );
        $ors   = array();

        $changed = $changed || count( $words ) > 1;

        foreach ( $words as $word ) {
            $like = '%' . $wpdb->esc_like( $word ) . '%';

            $ors[] = $wpdb->prepare( "({$wpdb->posts}.post_title LIKE %s) OR ({$wpdb->posts}.post_excerpt LIKE %s) OR ({$wpdb->posts}.post_content LIKE %s)", $like, $like, $like );
        }

        $parts[] = '(' . implode( ' OR ', $ors ) . ')';
    }

    if ( ! $changed ) {
        return $search;
    }

    $search = ' AND (' . implode( ' AND ', $parts ) . ') ';

    if ( ! is_user_logged_in() ) {
        $search .= " AND ({$wpdb->posts}.post_password = '') ";
    }

    return $search;
}, 10, 2 );