cannot be opened with mode "a"

Fatal error: Uncaught exception 'Zend_Log_Exception' with message '"/var/www/phpweb20/data/logs/debug.log" cannot be opened with mode "a"' in D:\AppServ\php6\library\Zend\Log\Writer\Stream.php:66 Stack trace: #0 D:\AppServ\www\php20\htdocs\index.php(12): Zend_Log_Writer_Stream->__construct('/var/www/phpweb...') #1 {main} thrown in D:\AppServ\php6\library\Zend\Log\Writer\Stream.php on line 66  Fatal error: Uncaught exception 'Zend_Log_Excep