php exec() - mysqldump creates an empty file

I want to create a backup from a database, but I get only a blank file.

include('config.php');

$command = "mysqldump --opt -h ".$_host." -u ".$_user." -p ".$_pass." ".$_db." > test.sql";
exec($command);

echo "<br />".$command;

test.sql is created where the .php file is located.

Edit:

Note! I'm using XAMPP WINDOWS !

Solution:

Because I'm using a Windows Web Server (XAMPP), I needed to specify the path:

$command = 'd:\xampp\mysql\bin\mysqldump --opt -u '.$_user.' -p'.$_pass.' '.$_db.' > test.sql';

1. I removed the space between the -p and the pass. It looks like: -pMYPASSWORD
2. Replaced " with '

I think if you are using a Linux based web server, you don't have to specify the path for mysqldump.

Cheers! :-)