To elaborate on rcrow's post, if you want to convert the objectSID value to a usable string (from Active Directory) the following function will do the trick (this was borrowed from another section of the manual, just thought I'd add it here):
// Returns the textual SID
function bin_to_str_sid($binsid) {
    $hex_sid = bin2hex($binsid);
    $rev = hexdec(substr($hex_sid, 0, 2));
    $subcount = hexdec(substr($hex_sid, 2, 2));
    $auth = hexdec(substr($hex_sid, 4, 12));
    $result    = "$rev-$auth";
    for ($x=0;$x < $subcount; $x++) {
        $subauth[$x] = 
            hexdec($this->little_endian(substr($hex_sid, 16 + ($x * 8), 8)));
        $result .= "-" . $subauth[$x];
    }
    // Cheat by tacking on the S-
    return 'S-' . $result;
}
// Converts a little-endian hex-number to one, that 'hexdec' can convert
function little_endian($hex) {
    for ($x = strlen($hex) - 2; $x >= 0; $x = $x - 2) {
        $result .= substr($hex, $x, 2);
    }
    return $result;
}
This function is not related to the ldap_get_values_len function but is still helpful if you want to convert the objectGUID binary value to a string format (converted from some vbscript provided by Richard Mueller):
// This function will convert a binary value guid into a valid string.
function bin_to_str_guid($object_guid) {
    $hex_guid = bin2hex($object_guid);
    $hex_guid_to_guid_str = '';
    for($k = 1; $k <= 4; ++$k) {
        $hex_guid_to_guid_str .= substr($hex_guid, 8 - 2 * $k, 2);
    }
    $hex_guid_to_guid_str .= '-';
    for($k = 1; $k <= 2; ++$k) {
        $hex_guid_to_guid_str .= substr($hex_guid, 12 - 2 * $k, 2);
    }
    $hex_guid_to_guid_str .= '-';
    for($k = 1; $k <= 2; ++$k) {
        $hex_guid_to_guid_str .= substr($hex_guid, 16 - 2 * $k, 2);
    }
    $hex_guid_to_guid_str .= '-' . substr($hex_guid, 16, 4);
    $hex_guid_to_guid_str .= '-' . substr($hex_guid, 20);
    return strtoupper($hex_guid_to_guid_str);
}
Here's an example on how to use both:
$filter="samaccountname=".$username;
$fields=array("objectguid","objectsid");
//establish the connection and specify the base_dn first. there are a lot of examples in the manual for this
$sr=ldap_search($this->_conn,$this->_base_dn,$filter,$fields);
$entries = ldap_get_entries($this->_conn, $sr);
if (in_array("objectguid", $fields)) {
    $entries[0]["objectguid"][0]=
        $this->bin_to_str_guid($entries[0]["objectguid"][0]);
}
if (in_array("objectsid", $fields)) {
    $entry = ldap_first_entry($this->_conn, $sr);
    $objectsid_binary = ldap_get_values_len($this->_conn, $entry, "objectsid");
    $entries[0]["objectsid"][0] = $this->bin_to_str_sid($objectsid_binary[0]);
}
Hope this helps someone!