PHP - Php And Eclipse

I wrote a scrit which mounts an external device, makes some backups then umount the device.
Everything works ok when I run the script under Eclipse PHP debugger but fails to unmount the device if I run it as localhost.

this is the code

Code: [Select]
$umount = '/bin/umount -f '.$drive;
....
u_mount($umount, $error=512);
....

This is the called function
Code: [Select]
function u_mount($cmd, $error) {
    $output = array();
    $return_var = 0;
    $return_var = exec($cmd, $output, $return_var);
    $return_var = $return_var + $error;
   .... some checks
}

In both cases (Eclipse and localhost) $return_var is always 0 but in localhost the umount is not executed and the device is still mounted

Any idea?

Thanks in advance
Gally