listaccts api search User by domain

Please can someone help me?
how to search the User by domain

example:

Code:

$domain = 'mydomain.com';

$accounts = $whm->listaccts($domain);

        foreach ($accounts as $ac){
              $userCp = $ac["user"];
        }

        echo $userCp;

I've tried, but it does not work.

Thanks,
Renata