Retrieves info for user lists to prevent multiple queries by get_userdata().

function cache_users( $user_ids ) {
		global $wpdb;

		update_meta_cache( 'user', $user_ids );

		$clean = _get_non_cached_ids( $user_ids, 'users' );

		if ( empty( $clean ) ) {
			return;
		}

		$list = implode( ',', $clean );

		$users = $wpdb->get_results( "SELECT * FROM $wpdb->users WHERE ID IN ($list)" );

		foreach ( $users as $user ) {
			update_user_caches( $user );
		}
	}
endif;

if ( ! function_exists( 'wp_mail' ) ) :
	/**
	 * Sends an email, similar to PHP's mail function.
	 *
	 * A true return value does not automatically mean that the user received the
	 * email successfully. It just only means that the method used was able to
	 * process the request without any errors.
	 *
	 * The default content type is `text/plain` which does not allow using HTML.
	 * However, you can set the content type of the email by using the
	 * {@see 'wp_mail_content_type'} filter.
	 *
	 * The default charset is based on the charset used on the blog. The charset can
	 * be set using the {@see 'wp_mail_charset'} filter.
	 *
	 * @since 1.2.1
	 * @since 5.5.0 is_email() is used for email validation,
	 *              instead of PHPMailer's default validator.
	 *
	 * @global PHPMailerPHPMailerPHPMailer $phpmailer
	 *
	 * @param string|string[] $to          Array or comma-separated list of email addresses to send message.
	 * @param string          $subject     Email subject.
	 * @param string          $message     Message contents.
	 * @param string|string[] $headers     Optional. Additional headers.
	 * @param string|string[] $attachments Optional. Paths to files to attach.
	 * @return bool Whether the email was sent successfully.
	 */