/**
  * Start the Session on request
  *
  */
 public static function start()
 {
     //=====
     //--
     if (self::$started !== false) {
         return;
         // avoid start session if already started ...
     }
     //end if
     self::$started = true;
     // avoid run start again
     //--
     //=====
     //--
     $browser_os_ip_identification = SmartUtils::get_os_browser_ip();
     // get browser and os identification
     //--
     if ((string) $browser_os_ip_identification['bw'] == '@s#' or (string) $browser_os_ip_identification['bw'] == 'bot') {
         return;
         // in this case start no session for robots or the self browser (as they do not need to share info between many visits) ; if the self browser fail to identify will be at least identified as robot in the worst case
     }
     //end if
     //--
     //=====
     //-- no log as the cookies can be dissalowed by the browser
     if ((string) SMART_APP_VISITOR_COOKIE == '') {
         return;
         // session need cookies
     }
     //end if
     //--
     //=====
     //--
     $sf_sess_mode = 'files';
     $sf_sess_area = 'default-sess';
     $sf_sess_ns = 'unknown';
     $sf_sess_dir = 'tmp/sess';
     //--
     //=====
     if (!defined('SMART_FRAMEWORK_SESSION_PREFIX')) {
         Smart::log_warning('FATAL ERROR: Invalid Session Prefix :: SMART_FRAMEWORK_SESSION_PREFIX');
         return;
     }
     //end if
     if (strlen(SMART_FRAMEWORK_SESSION_PREFIX) < 3 or strlen(SMART_FRAMEWORK_SESSION_PREFIX) > 9) {
         Smart::log_warning('WARNING: Session Prefix must have a length between 3 and 9 characters :: SMART_FRAMEWORK_SESSION_PREFIX');
         return;
     }
     //end if
     if (!preg_match('/^[a-z\\-]+$/', (string) SMART_FRAMEWORK_SESSION_PREFIX)) {
         Smart::log_warning('WARNING: Session Prefix contains invalid characters :: SMART_FRAMEWORK_SESSION_PREFIX');
         return;
     }
     //end if
     //--
     if (!defined('SMART_FRAMEWORK_SESSION_NAME')) {
         Smart::log_warning('FATAL ERROR: Invalid Session Name :: SMART_FRAMEWORK_SESSION_NAME');
         return;
     }
     //end if
     if (strlen(SMART_FRAMEWORK_SESSION_NAME) < 10 or strlen(SMART_FRAMEWORK_SESSION_NAME) > 25) {
         Smart::log_warning('WARNING: Session Name must have a length between 10 and 25 characters :: SMART_FRAMEWORK_SESSION_NAME');
         return;
     }
     //end if
     if (!preg_match('/^[_A-Za-z0-9]+$/', (string) SMART_FRAMEWORK_SESSION_NAME)) {
         Smart::log_warning('WARNING: Session Name contains invalid characters :: SMART_FRAMEWORK_SESSION_NAME');
         return;
     }
     //end if
     if (!SmartFrameworkSecurity::ValidateVariableName(strtolower(SMART_FRAMEWORK_SESSION_NAME))) {
         Smart::log_warning('WARNING: Session Name have an invalid value :: SMART_FRAMEWORK_SESSION_NAME');
         return;
     }
     //end if
     //--
     if (!defined('SMART_FRAMEWORK_SESSION_LIFETIME')) {
         Smart::log_warning('FATAL ERROR: Invalid Session GC Lifetime :: SMART_FRAMEWORK_SESSION_LIFETIME');
         return;
     }
     //end if
     if (!is_int(SMART_FRAMEWORK_SESSION_LIFETIME)) {
         Smart::log_warning('Invalid INIT constant value for SMART_FRAMEWORK_SESSION_LIFETIME');
         return;
     }
     //end if
     //--
     if (!is_dir('tmp/sessions/')) {
         Smart::log_warning('FATAL ERROR: The Folder \'tmp/sessions/\' does not exists for use with Session !');
         return;
     }
     //end if
     //--
     $detected_session_mode = (string) ini_get('session.save_handler');
     if ((string) $detected_session_mode === 'files') {
         if ((string) SMART_FRAMEWORK_SESSION_HANDLER !== 'files') {
             Smart::log_warning('FATAL ERROR: The value set for SMART_FRAMEWORK_SESSION_HANDLER is not set to: files / but the value found in session.save_handler is: ' . $detected_session_mode);
             return;
         }
         //end if
     } elseif ((string) $detected_session_mode === 'user') {
         if ((string) SMART_FRAMEWORK_SESSION_HANDLER === 'files') {
             Smart::log_warning('FATAL ERROR: The value set for SMART_FRAMEWORK_SESSION_HANDLER is set to: files / but the value found in session.save_handler is: ' . $detected_session_mode);
             return;
         }
         //end if
     } else {
         Smart::log_warning('FATAL ERROR: The value set for session.save_handler must be set to one of these modes: files or user');
         return;
     }
     //end if
     //--
     //=====
     //--  generate a the client private key based on it's IP and Browser
     $the_sess_client_uuid = SmartUtils::unique_client_private_key();
     // SHA512 key to protect session data agains forgers
     //-- a very secure approach based on a chain, derived with a secret salt from the framework security key:
     // (1) an almost unique client private key lock based on it's IP and Browser
     // (2) an entropy derived from the client random cookie combined with the (1)
     // (3) a unique session name suffix derived from (1) and (2)
     // (4) a unique session id composed from (1) and (2)
     //-- thus the correlation between the random public client cookie, the session name suffix and the session id makes impossible to forge it as it locks to IP+Browser, using a public entropy cookie all encrypted with a secret key and derived and related, finally composed.
     $the_sess_client_lock = SmartHashCrypto::sha1(SMART_FRAMEWORK_SECURITY_KEY . '#' . $the_sess_client_uuid);
     $the_sess_client_entropy = SmartHashCrypto::sha1(SMART_APP_VISITOR_COOKIE . '*' . $the_sess_client_uuid . '%' . SMART_FRAMEWORK_SECURITY_KEY);
     $the_sess_nsuffix = SmartHashCrypto::sha1($the_sess_client_uuid . ':' . SMART_FRAMEWORK_SECURITY_KEY . '^' . $the_sess_client_entropy . '+' . $the_sess_client_lock . '$' . SMART_APP_VISITOR_COOKIE);
     $the_sess_id = $the_sess_client_entropy . '-' . $the_sess_client_lock;
     // session ID combines the secret client key based on it's IP / Browser and the Client Entropy Cookie
     //--
     $sf_sess_area = Smart::safe_filename((string) SMART_FRAMEWORK_SESSION_PREFIX);
     $sf_sess_dpfx = substr($the_sess_client_entropy, 0, 1) . '-' . substr($the_sess_client_lock, 0, 1);
     // this come from hexa so 3 chars are 16x16x16=4096 dirs
     //--
     if ((string) $browser_os_ip_identification['bw'] == '@s#') {
         $sf_sess_ns = '@sr-' . $sf_sess_dpfx;
     } elseif ((string) $browser_os_ip_identification['bw'] == 'bot') {
         $sf_sess_ns = 'r0-' . $sf_sess_dpfx;
         // we just need a short prefix for robots (on disk is costly for GC to keep separate folders, but of course, not so safe)
     } else {
         $sf_sess_ns = 'c-' . substr($browser_os_ip_identification['bw'], 0, 3) . '-' . $sf_sess_dpfx;
         // we just need a short prefix for clients (on disk is costly for GC to keep separate folders, but of course, not so safe)
     }
     //end if else
     $sf_sess_ns = Smart::safe_filename($sf_sess_ns);
     //-- by default set for files
     $sf_sess_mode = 'files';
     $sf_sess_dir = 'tmp/sessions/' . $sf_sess_area . '/' . $sf_sess_ns . '/';
     if ((string) $detected_session_mode === 'user') {
         if (class_exists('SmartCustomSession')) {
             if ((string) get_parent_class('SmartCustomSession') == 'SmartAbstractCustomSession') {
                 $sf_sess_mode = 'user-custom';
                 $sf_sess_dir = 'tmp/sessions/' . $sf_sess_area . '/';
                 // here the NS is saved in DB so we do not need to complicate paths
             } else {
                 Smart::log_warning('SESSION INIT ERROR: Invalid Custom Session Handler. The class SmartCustomSession must be extended from class SmartAbstractCustomSession ...');
                 return;
             }
             //end if else
         } else {
             Smart::log_warning('SESSION INIT ERROR: Custom Session Handler requires the class SmartCustomSession ...');
             return;
         }
         //end if
     }
     //end if
     $sf_sess_dir = Smart::safe_pathname($sf_sess_dir);
     //--
     if (!is_dir($sf_sess_dir)) {
         SmartFileSystem::dir_recursive_create($sf_sess_dir);
     }
     //end if
     SmartFileSystem::write_if_not_exists('tmp/sessions/' . $sf_sess_area . '/' . 'index.html', '');
     //=====
     //--
     @session_save_path($sf_sess_dir);
     @session_cache_limiter('nocache');
     //--
     $the_name_of_session = (string) SMART_FRAMEWORK_SESSION_NAME . '__Key_' . $the_sess_nsuffix;
     // protect session name data agains forgers
     //--
     @session_id((string) $the_sess_id);
     @session_name((string) $the_name_of_session);
     //--
     $tmp_exp_seconds = Smart::format_number_int(SMART_FRAMEWORK_SESSION_LIFETIME, '+');
     if ($tmp_exp_seconds > 0) {
         @session_set_cookie_params((int) $tmp_exp_seconds, '/');
         // session cookie expire and the path
     }
     // end if
     //-- be sure that session_write_close() is executed at the end of script if script if die('') premature and before pgsql shutdown register in the case of DB sessions
     register_shutdown_function('session_write_close');
     //-- handle custom session handler
     if ((string) $sf_sess_mode === 'user-custom') {
         //--
         $sess_obj = new SmartCustomSession();
         $sess_obj->sess_area = (string) $sf_sess_area;
         $sess_obj->sess_ns = (string) $sf_sess_ns;
         $sess_obj->sess_expire = (int) $tmp_exp_seconds;
         //--
         session_set_save_handler(array($sess_obj, 'open'), array($sess_obj, 'close'), array($sess_obj, 'read'), array($sess_obj, 'write'), array($sess_obj, 'destroy'), array($sess_obj, 'gc'));
         //--
     }
     //end if else
     //-- start session
     @session_start();
     //--
     if ((string) $_SESSION['SoftwareFramework_VERSION'] != (string) SMART_FRAMEWORK_VERSION or (string) $_SESSION['website_ID'] != (string) SMART_SOFTWARE_NAMESPACE or strlen($_SESSION['session_ID']) < 32) {
         //--
         $_SESSION['SoftwareFramework_VERSION'] = (string) SMART_FRAMEWORK_VERSION;
         // software version
         $_SESSION['SoftwareFramework_SessionMode'] = (string) $sf_sess_mode;
         // session mode
         $_SESSION['website_ID'] = (string) SMART_SOFTWARE_NAMESPACE;
         // the website ID
         $_SESSION['uniqbrowser_ID'] = (string) $the_sess_client_uuid;
         // a true unique browser ID (this is a protection against sessionID forgers)
         $_SESSION['session_ID'] = (string) @session_id();
         // read current session ID
         $_SESSION['session_STARTED'] = (string) date('Y-m-d H:i:s O');
         // read current session ID
         //--
     }
     //end if
     //--
     if (!isset($_SESSION['visit_COUNTER'])) {
         $_SESSION['visit_COUNTER'] = 1;
     } else {
         $_SESSION['visit_COUNTER'] += 1;
     }
     //end if else
     //--
     $_SESSION['SmartFramework__Browser__Identification__Data'] = (array) $browser_os_ip_identification;
     //--
     if ((string) $_SESSION['uniqbrowser_ID'] != (string) $the_sess_client_uuid) {
         // we need at least a md5 session
         //-- log, then unset old session (these are not well tested ...)
         Smart::log_notice('Session Security Breakpoint :: Session-BrowserUniqueID = ' . $_SESSION['uniqbrowser_ID'] . "\n" . 'SessionSecurityUniqueID = ' . $the_sess_client_uuid . "\n" . 'Browser Ident = ' . $browser_os_ip_identification['bw'] . "\n" . 'Cookies = ' . print_r($_COOKIE, 1) . "\n" . 'SessID = ' . $_SESSION['session_ID'] . "\n" . 'ClientIP = ' . SmartUtils::get_ip_client() . ' @ ' . $_SERVER['REMOTE_ADDR'] . "\n" . 'UserAgent = ' . $_SERVER['HTTP_USER_AGENT']);
         $_SESSION = array();
         // reset it
         //-- unset the cookie (from this below is tested)
         @setcookie($the_name_of_session, 'EXPIRED', 1, '/');
         //-- stop execution with message
         Smart::raise_error('SESSION // SECURITY BREAK POINT: Possible Session Forgery Detected ...', 'SESSION // SECURITY BREAK POINT: Possible Session Forgery Detected ! Please refresh the page ... A new session will be assigned ! If you are not trying to forge another user\' session this situation can occur also if you are behind a proxy and some of your navigation parameters has been changed ! If this problem persist try to restart your browser or use other browser. If still persist, contact the website administrator');
         die('');
         // just in case
         return;
         // or is better to silent discard it ?
         //--
     }
     //end if
     //--
     self::$active = time();
     // successfuly started
     //--
 }