PHP for custom user fields in directory path

Hi, I am trying to do a similar thing to this post http://drupal.org/node/416438

I have created custom fields under the account settings, for example the machine name is field_account_number.

When I enter the directory path as the following I get a fatal error message.
php: if (!isset($user->field_account_number)) profile_load_profile($user); return 'images/' .$user->field_account_number;