ldap用户“部门”的下拉列表

I'm still new in PHP, How do i "GROUP BY" all "department" in Active Directory and make it a drop down list selection on PHP? I have already tried to list down that string, but it to many duplicates "department". Thank you.

** UPDATE! This is my code.

//FILTER FOR USER ONLY
$object_filter  ='(&(objectClass=user)(objectCategory=person)(!(UserAccountControl:1.2.840.113556.1.4.803:=2)))';
$ldap_search    = ldap_search($ldap_connect, $ou, $object_filter);

//SORT NAME FROM A - Z
ldap_sort  ($ldap_connect  , $ldap_search  , "cn");

$ldap_get_entries = ldap_get_entries($ldap_connect, $ldap_search);  

// No information found, bad user
if($ldap_get_entries['count'] == 0) return false;

echo '<select>';
for ($i=0; $i < $ldap_get_entries["count"]; $i++){
    $department = $ldap_get_entries[$i]["department"][0];
    echo '<option value=' . $department . '>' . $department . '</option>';
}
echo '</select>';