Getting Backup diskspace value

Good Day happy people.
Please I have been trying to get the server backup-disk space from a cpanel account with the below php code, but I keep getting error:
code:
$df = disk_free_space("/backup");
echo $df;

Error:
PHP Warning: disk_free_space(): No such file or directory in /home/username/public_html/test.php on line 137

I have added the cpanel user to the root group(I have root previlage ), the "/backup" drive permission is 0771 and the user can execute exec() command.

Kindly help.

 

Yes Sir, it does

 

Thanks for your response.
Yes the code works for "/" directory or "/home" which is where the user account resides.
Thanks.

 

Thanks for your response.
We have several severs that their backup diskspace fills-up easily due to their size. The management is not ready to upgrade the backup diskspace, so most time we only get to know the disk is filled-up when we try to restore an account from back-up and a diskspace error comes up. In essence, I want to get the backup disk value s from all our servers and display all of them on a single page.

P.S Thanks for the UAPI function reference, but my focus is on the backup disk value for the whole server not users.