php exec mysqldump to back up database in sql format

I'm trying to use exec() in php to run mysqldump to back up a database in AWS(Amazon Web Service) with name projectdata. But I can only create an empty sql file.

I'm running the php file with xampp, under Windows 7 where mysqldump is in C:\xampp\mysql\mysqldump

Please help :

> exec('C:\xampp\mysql\mysqldump --user=user --password=password --host=cannotTellyou.amazonaws.com:3306 projectdata  > backup.sql');

Thanks for your attention.