ロジック忘れないように忘備録
functions.phpなどに
/**
 * 検索ロジックカスタマイズ
 *
 */
function customSearcher($search, $wp_query) {
	if ( $wp_query->is_main_query() && $wp_query->is_search() && !is_admin() ) {
		$q = $wp_query->query_vars;
		$n = null;
		if (!empty($q['s'])) {

			$keyword = mb_convert_kana($q['s'], 's');
			$keywords = preg_split('/[\s]+/', $keyword, -1, PREG_SPLIT_NO_EMPTY);

			foreach ($keywords as $word) {
				$n = " OR ( pm.meta_key IN ( 'search_data' ) AND pm.meta_value LIKE '%" . $word . "%') ";

				$search = str_replace( "post_content LIKE '%". $word .  "%')", "post_content LIKE '%". $word .  "%')" . $n , $search );
			}

		}

	}
	return $search;
}
add_filter('posts_search', 'customSearcher', 10, 2);