WCF::handleException() Unhandled exception: Class "PDO" not found

#0 /home/user/public_html/wcf/lib/system/database/Database.class.php(107): wcf\system\database\MySQLDatabase->connect()
#1 /home/user/public_html/wcf/lib/system/WCF.class.php(324): wcf\system\database\Database->__construct(...)
#2 /home/user/public_html/wcf/lib/system/WCF.class.php(141): wcf\system\WCF->initDB()
#3 /home/user/public_html/wcf/global.php(25): wcf\system\WCF->__construct()
#4 /home/user/public_html/global.php(13): require_once('/home/user/publ...')
#5 /home/user/public_html/index.php(12): require_once('/home/user/publ...')
#6 {main}