Fatal error: PHP error in file /var/www/web649/html/board/wcf/lib/page/UserGuestbookPage.class.php (110): Non-static method UserGuestbookData::getStats() should not be called statically, assuming $this from incompatible context

Information:

error message: PHP error in file /var/www/web649/html/board/wcf/lib/page/UserGuestbookPage.class.php (110): Non-static method UserGuestbookData::getStats() should not be called statically, assuming $this from incompatible context
error code: 0
file: /var/www/web649/html/board/wcf/lib/system/WCF.class.php (255)
php version: 5.6.40
wcf version: 1.0.11 pl 4 (Horizon)
date: Fri, 18 Oct 2019 17:07:36 +0000
request: /index.php?page=UserGuestbook&userID=27&s=11f834436e588ef6797afe3e26f723fa73e688cd
referer:

Stacktrace:

#0 /var/www/web649/html/board/wcf/lib/page/UserGuestbookPage.class.php(110): WCF::handleError(8192, 'Non-static meth...', '/var/www/web649...', 110, Array)
#1 /var/www/web649/html/board/wcf/lib/page/AbstractPage.class.php(74): UserGuestbookPage->readData()
#2 /var/www/web649/html/board/wcf/lib/page/UserGuestbookPage.class.php(184): AbstractPage->show()
#3 /var/www/web649/html/board/wcf/lib/page/AbstractPage.class.php(36): UserGuestbookPage->show()
#4 /var/www/web649/html/board/wcf/lib/page/UserGuestbookPage.class.php(28): AbstractPage->__construct()
#5 /var/www/web649/html/board/wcf/lib/util/RequestHandler.class.php(63): UserGuestbookPage->__construct()
#6 /var/www/web649/html/board/wcf/lib/util/RequestHandler.class.php(91): RequestHandler->__construct('UserGuestbook', Array, 'page')
#7 /var/www/web649/html/board/index.php(8): RequestHandler::handle(Array)
#8 {main}