Problem with GLPI session file path

Configuration:

GLPI 0.83.8
HHVM 3.7.0
Apache 2.4
Debian 8 (jessie)

Problem:

Can't log into GLPI: the login page is shown again and again with no error nowhere...

Cause:

The session file path is not accessible by GLPI due to relative GLPI_ROOT definition.

Fix:

Set GLPI_SESSION_DIR to an absolute path instead of relative path.
Edit config/based_config.php and replace line:

define("GLPI_SESSION_DIR",GLPI_ROOT . "/files/_sessions");

by
define("GLPI_SESSION_DIR","/var/www/html/processmaker/workflow/public_html/glpi/files/_sessions");

Of course use your own path to the session file folder.