What article on PHPBuilder said to explicitly call the garbage collector?
The garbage collection function simply cleans things up by deleting old, expired session data. There should be no reason to call this function in your script as it's only purpose is to free up dead space. The session data that WOULD be deleted should be marked as expired anyway, so those sessions should NOT be open-able or activated by your script.
When writing custome session handlers and storing session data in a database, the garbage collection function should be a simple query similar to the following:
$query = "DELETE FROM session_data WHERE expires < " . time();