WCF::handleException() Unhandled exception: Undefined class constant 'MYSQL_ATTR_INIT_COMMAND'

#0 /www/htdocs/w0099e12/forum/wcf/lib/system/database/Database.class.php(107): wcf\system\database\MySQLDatabase->connect()
#1 /www/htdocs/w0099e12/forum/wcf/lib/system/WCF.class.php(324): wcf\system\database\Database->__construct(...)
#2 /www/htdocs/w0099e12/forum/wcf/lib/system/WCF.class.php(141): wcf\system\WCF->initDB()
#3 /www/htdocs/w0099e12/forum/wcf/global.php(25): wcf\system\WCF->__construct()
#4 /www/htdocs/w0099e12/forum/gallery/global.php(13): require_once('/www/htdocs/w00...')
#5 /www/htdocs/w0099e12/forum/gallery/index.php(9): require_once('/www/htdocs/w00...')
#6 {main}