Symptoms: After calling session_start() and assigning key-value pairs to $_SESSION, calling print_r($_SESSION) on subsequent pages result in an empty array; i.e. $_SESSION values are not retained across pages.
Solution: Assuming your php.ini (session) save_handler, save_path variables are set correctly, also check if sufficient disk space is available.
I spent 90 minutes trying to resolve the problem thinking it was a PHP-related issue, but in the end, nullifying the 5+ GiB of httpd error logs promptly resolved the problem. Doh!