Deletes all expired transients.

Note that this function won’t do anything if an external object cache is in use.

The multi-table delete syntax is used to delete the transient record
from table a, and the corresponding transient_timeout record from table b.

function delete_expired_transients( $force_db = false ) {
	global $wpdb;

	if ( ! $force_db && wp_using_ext_object_cache() ) {
		return;
	}

	$wpdb->query(
		$wpdb->prepare(
			"DELETE a, b FROM {$wpdb->options} a, {$wpdb->options} b
			WHERE a.option_name LIKE %s
			AND a.option_name NOT LIKE %s
			AND b.option_name = CONCAT( '_transient_timeout_', SUBSTRING( a.option_name, 12 ) )
			AND b.option_value esc_like( '_transient_' ) . '%',
			$wpdb->esc_like( '_transient_timeout_' ) . '%',
			time()
		)
	);

	if ( ! is_multisite() ) {
		// Single site stores site transients in the options table.
		$wpdb->query(
			$wpdb->prepare(
				"DELETE a, b FROM {$wpdb->options} a, {$wpdb->options} b
				WHERE a.option_name LIKE %s
				AND a.option_name NOT LIKE %s
				AND b.option_name = CONCAT( '_site_transient_timeout_', SUBSTRING( a.option_name, 17 ) )
				AND b.option_value esc_like( '_site_transient_' ) . '%',
				$wpdb->esc_like( '_site_transient_timeout_' ) . '%',
				time()
			)
		);
	} elseif ( is_multisite() && is_main_site() && is_main_network() ) {
		// Multisite stores site transients in the sitemeta table.
		$wpdb->query(
			$wpdb->prepare(
				"DELETE a, b FROM {$wpdb->sitemeta} a, {$wpdb->sitemeta} b
				WHERE a.meta_key LIKE %s
				AND a.meta_key NOT LIKE %s
				AND b.meta_key = CONCAT( '_site_transient_timeout_', SUBSTRING( a.meta_key, 17 ) )
				AND b.meta_value esc_like( '_site_transient_' ) . '%',
				$wpdb->esc_like( '_site_transient_timeout_' ) . '%',
				time()
			)
		);
	}
}