searchwp_include

<?php
function my_searchwp_include( $ids ) {
    if ( isset( $_REQUEST['country'] ) ) {
        // limit the results to all of the chosen countries
        $args = array(
            'post_type' => 'any',
            'nopaging' => true,
            'fields' => 'ids',
            'tax_query' => array(
                array(
                    'taxonomy' => 'country',
                    'field'    => 'slug',
                    'terms'    => $_REQUEST['country'],
                ),
            ),
        );
        $country_ids = get_posts( $args );
        $c_ids = array_merge( $ids, $country_ids );
        $c_ids = array_unique( $c_ids );
    }
    if ( isset( $_REQUEST['soort'] ) ) {
        // limit the results to all of the chosen countries
        $args = array(
            'post_type' => 'any',
            'nopaging' => true,
            'fields' => 'ids',
            'tax_query' => array(
                array(
                    'taxonomy' => 'soort',
                    'field'    => 'slug',
                    'terms'    => $_REQUEST['soort'],
                ),
            ),
        );
        $soort_ids = get_posts( $args );
        $s_ids = array_merge( $ids, $soort_ids );
        $s_ids = array_unique( $s_ids );
    }
    if ( isset( $_REQUEST['thema'] ) ) {
        // limit the results to all of the chosen countries
        $args = array(
            'post_type' => 'any',
            'nopaging' => true,
            'fields' => 'ids',
            'tax_query' => array(
                array(
                    'taxonomy' => 'thema',
                    'field'    => 'slug',
                    'terms'    => $_REQUEST['thema'],
                ),
            ),
        );
        $thema_ids = get_posts( $args );
        $t_ids = array_merge( $ids, $thema_ids );
        $t_ids = array_unique( $t_ids );
    }

    $ids = array_merge( $ids, $country_ids, $soort_ids, $thema_ids );
    return $ids;
}
add_filter( 'searchwp_include', 'my_searchwp_include' );