using write_file()

I am using write_file to to write out a backup of my sql tables once/week:


$backup = $this->dbutil->backup($prefs);
write_file('/opt/bitnami/mysql/tmp/mybackup.gz', $backup)

$source = '/opt/bitnami/mysql/tmp/mybackup.gz';

// Where the files will be transferred to
           $dest = 's3://substantiator-mysql-backup';

// Create a default transfer object
           $manager = new \Aws\S3\Transfer($client, $source, $dest);

 
This seems to write out a long string of data to the file named above. The problem is that when i try uploading this to Amazon I get an error that the function can't find /opt/bitnami/mysql/tmp/mybackup.gz .

I checked the permissions of that file and I see it is: 660. 

Is the problem in the way the backup was created? Or perhaps the permissions on the /tmp directory?

proof that an old dog can learn new tricks