How to get the DN for a given samaccountname:
/*the connecting part*/
$ldap_conn = ldap_connect($hostname, $port);
$bind = ldap_bind($ldap_conn, $username, $password);
/*the connecting part*/
$samaccountname = john.doe;
$filter="(samaccountname=$samaccountname)";
$dn="OU=PEOPLE, DC=example, DC=com"; //even if it seems obvious I note here that the dn is just an example, you'll have to provide an OU and DC of your own
$res = ldap_search($ldap_conn, $dn, $filter);
$first = ldap_first_entry($ldap_conn, $res);
$data = ldap_get_dn($ldap_conn, $first);
echo "The desired DN is: ".$data;