They may not have register globals running and if they have a version of php below 4.3 I believe you can't use the following but if they do try this.
//Registering Sessions without globals
Session_start();
$_SESSION["variable_name"] = $variable;
//Reading the registered variable later
session_start();
$variable = $_SESSION["variable_name"];
This is the only thing I can think of but I recommend also doing a search of the forums for IIS + Sessions