object persistence within a session

If this is the wrong forum, please point me at the correct forum. I am new to PHP but have 40 years experience programming. My initial effort includes a class definition which needs to persist for the duration of a WWW session. The code (this snippet is the beginning of Default.php) <?PHP session_start(); require_once 'CSSFrames_Includes/Classes.inc'; if (!isset($_SESSION["navigator"])) { $_SESSION["navigator"] = new CSSFrames_Navigator; } else { parse_str($_SERVER['QUERY_STRING'], $queryString); foreach ($queryString as $key => $value) { switch($key) { case "ID": $_SESSION["navigator"]->set_page_ID($value); break; default: break; } } } initially works ... $_session["navigator"] does exist and methods do function correctly. When, however, I click a link and cause the script (Default.php) to be reentered, I get the following Fatal error: main() [<a href='function.main'>function.main</a>]: The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition &quot;MyClass_defined&quot; of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition in /MyWebSite.com/Default.php on line 16 which I understand to mean that my class definition (file?) has not been loaded. What I do not understand is why has not the class definition been loaded? Did storing the object reference not also store the class definition? What must I do to retain the class definition as part of the session data? Thanks, Stan