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/**
 * postのみ検索結果に表示
 *
 * @param $search
 * @param $wp_query
 * @return string|void
 */
add_filter( 'posts_search', function ( $search, $wp_query ) {
	if ( !$wp_query->is_search ) {
		return '';
	}
	$search .= " AND post_type = 'post'";
	return $search;
}, 10, 2 );