mysqldump via PHP
===

I have a PHP script that gets passed the MySQL connection details of a remote server and I want it to execute a mysqldump command. To do this I'm using the php exec() function:

    <?php
    exec("/usr/bin/mysqldump -u mysql-user -h 123.145.167.189 -pmysql-pass database_name > /path-to-export/file.sql", $output);
    ?>

When the right login details are passed to it, it'll work absolutely fine.
However, I'm having trouble checking *if* it executes as expected and if it doesn't finding out why not.
The $output array returns as empty, whereas if I run the command directly on the command line a message is printed out telling me the login failed. I want to capture such error messages and display them. Any ideas on how to do that?