Can't preset duration value with php code

Hello.

I can't preset duration value with php code.

The hint in the interface says:

return array(
0 => array('iso8601' => value for iso8601, 'approx_seconds' => value for approx_seconds),
// You'll usually want to stop here. Provide more values
// if you want your 'default value' to be multi-valued:
1 => array('iso8601' => value for iso8601, 'approx_seconds' => value for approx_seconds),
2 => ...
);

And it does not work.

How would a proper array look to preset the value?