I was unable to recreate your issue. My understanding of the code is as following.
PARAM_NAME_API_KEY, PARAM_NAME_USER_NAME, and PARAM_NAME_PASSWORD is define as a Constant String in Constants.php
The Value of PARAM_NAME_API_KEY equals to "api_key"
The Value of PARAM_NAME_USER_NAME equals to "site_owner_user_name"
The Value of PARAM_NAME_PASSWORD equals to "sopassword'
This means that this code in GetList.php
$xml = executeRequest(URI . '/ws/customers/' . $_REQUEST . '/lists');
would give you the same results as this code
$xml = executeRequest(URI . '/ws/customers/' . $_REQUEST . '/lists');
Also this code in Functions.php,
$userNamePassword = $_REQUEST . '%' . $_REQUEST . ':' . $_REQUEST;
would give you the same results as this code
$userNamePassword = $_REQUEST . '%' . $_REQUEST . ':' . $_REQUEST;
This should not have worked for you in Function.php
$userNamePassword = PARAM_NAME_API_KEY . '%' . PARAM_NAME_USER_NAME . ':' . PARAM_NAME_PASSWORD;
If you echo $userNamePassword after using the above you would get this string
"api_key%site_owner_user_name:sopassword"
It would not echo your actual API Key, Username or Password, but rather the names of the variables you are using
Mike C Support Engineer, Constant Contact