hidden profile fields showing up, and what exactly does "visibility != %d" mean?

I am working with an organization that has a big membership/donations/contacts database and they wanted a way to link up drupal user accounts with their database. I wanted to add a field to the drupal profile to store the ID from the database, but I didn't want a non-admin to see this field or to be able to change this field's value. A visiblity option of 'Admin Only' would seem to suffice for my purposes...

There is a similar discussion about this at node/12737 with a patch, but I tried it, and it didn't achieve the effect that I was looking for, so I am not sure what that patch actually does.

Then I found this post, and it seemed that this was a good start, but I needed a little more.

Things you should know...
1. I am new to drupal
2. These code changes are made to the profile module which is a core module, so I was a bit hesitant...
3. Please make a backup copy of the profile module, so you can switch back, in case these changes don't work for you because of some other dependency.
4. I don't know how to create a patch so the changes I made are documented here and you will have to make them yourself manually (if someone wanted to make a patch, that would be great!).
5. You should not have a profile category that displays only admin fields, as the link will show up at the top when a user chooses to Edit his/her profile (no fields will show if the user clicks that category, the field section will be blank)
6. The two options "The user must enter a value." and "Visible in user registration form." should be left un-checked for these admin fields.
7. Applies to 4.6 // $Id: profile.module,v 1.90.2.2 2005/06/01 04:30:07 unconed Exp $

So here are the changes I made...

Near the top (@ line 12) 3 values are defined, define a 4th value for admin only ...
After this section

/**
 * Flags to define the visibility of a profile field.
 */

Add the following 1 line

define('PROFILE_ADMIN', 0);

In the function profile_browse ...
Change this line

if (!user_access('administer users') && $field->visibility != PROFILE_PRIVATE) {

To the following 1 line

if (!user_access('administer users') && $field->visibility <= PROFILE_PRIVATE) {

In the function profile_view_field ...
Change this line

$browse = user_access('administer users') || $field->visibility == PROFILE_PRIVATE;

To the following 1 line

$browse = user_access('administer users') || $field->visibility > PROFILE_PRIVATE;

In the function profile_view_profile ...
Change these 6 lines

if (user_access('administer users') || $GLOBALS['user']->uid == $user->uid) {
  $result = db_query('SELECT * FROM {profile_fields} ORDER BY category, weight');
}
else {
  $result = db_query('SELECT * FROM {profile_fields} WHERE visibility != %d ORDER BY category, weight', PROFILE_PRIVATE);
}

To the following 9 lines

if (user_access('administer users')) {
  $result = db_query('SELECT * FROM {profile_fields} ORDER BY category, weight');
}
elseif ($GLOBALS['user']->uid == $user->uid) {
  $result = db_query('SELECT * FROM {profile_fields} WHERE visibility >= %d ORDER BY category, weight', PROFILE_PRIVATE);
}
else {
  $result = db_query('SELECT * FROM {profile_fields} WHERE visibility > %d ORDER BY category, weight', PROFILE_PRIVATE);
}

In the same function ...
Change this line

$description = ($field->visibility == PROFILE_PRIVATE) ? t('The content of this field is private and only visible to yourself.') : '';

To these 2 lines

$description = ($field->visibility == PROFILE_ADMIN) ? t('The content of this field is visible to administrators only.') : '';
$description = ($field->visibility == PROFILE_PRIVATE) ? t('The content of this field is private and only visible to yourself.') : $description;

In the function _profile_form_explanation ...
After this section

if ($field->visibility == PROFILE_PRIVATE) {
  $output .= ' '. t('The content of this field is kept private and will not be shown publicly.');
}

Add the following 3 lines

if ($field->visibility == PROFILE_ADMIN) {
  $output .= ' '. t('The content of this field is viewable by administatators only.');
}

In the function profile_form_profile ...
Change this line

$result = db_query("SELECT * FROM {profile_fields} WHERE LOWER(category) = LOWER('%s') ORDER BY weight", $category);

To these 6 lines

if (user_access('administer users')) {
    $result = db_query("SELECT * FROM {profile_fields} WHERE LOWER(category) = LOWER('%s') ORDER BY weight", $category);
}
elseif ($GLOBALS['user']->uid == $user->uid) {
    $result = db_query("SELECT * FROM {profile_fields} WHERE LOWER(category) = LOWER('%s') AND visibility >= %d ORDER BY weight", $category, PROFILE_PRIVATE);
}    

In the function _profile_field_form ...
Change this line

$group .= form_radios(t('Visibility'), 'visibility', $edit['visibility'], array(PROFILE_PRIVATE => t('Private field, content only available to privileged users.'), PROFILE_PUBLIC => t('Public field, content shown on profile page but not used on member list pages.'), PROFILE_PUBLIC_LISTINGS => t('Public field, content shown on profile page and on member list pages.')));

To this line

$group .= form_radios(t('Visibility'), 'visibility', $edit['visibility'], array(PROFILE_ADMIN => t('Administrator field, content only available to administrators.'), PROFILE_PRIVATE => t('Private field, content only available to privileged users.'), PROFILE_PUBLIC => t('Public field, content shown on profile page but not used on member list pages.'), PROFILE_PUBLIC_LISTINGS => t('Public field, content shown on profile page and on member list pages.')));