I have problems with reading users from an LDAP, if this exists in a different domain.

From the programming environment and forth is "my" system similar to the infos under php.net. Users who are under the same domain in which I am, I can query. Users who are outside, I can not read.
Can anyone make a rhyme of it?

Even the technical background:
The server (MS IIS) and also of users, under which the connection is established, are in the same domain. Also, the LDAP server is located in this domain.
I have only the CN of the user.
An entry in to search Domian looks like like this:
PHP code:
PHP Code:
dn "dc = aa001, DC = company, dc = net"
or PHP code:
PHP Code:
$dn "DC=eu001,DC=firma,DC=net"