Session are not available in site.php (config file)

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Hello everyone,

When i try to access session from site.php of config the session are not available over there, So i tried to do session_start() from site.php but that is giving me error as below :

Fatal error: Concrete5_Model_PermissionAccess::validateAndFilterAccessEntities(): The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "GroupPermissionAccessEntity" 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 /home/c2085051/public_html/updates/concrete5.6.0.2/concrete/core/models/permission/access/model.php on line 51

So what wrong i am doing here ? And what is the way with which i can access the session from site.php, please help to get me solution of it