WordPress Advanced Search Example

functions.php

<?php

function humus_advSearch_query($query) {

	if(is_search() && $_REQUEST['tags']) {
		$tax_query = array(
			'taxonomy' => 'post_tag',
			'terms' => $_REQUEST['tags'],
			'field' => 'slug'
		);
		$query->set('tax_query', $tax_query);
	}

}

add_action('pre_get_posts', 'humus_advSearch_query');

?>