session_name() returns the name of the current session.
The session name is reset to the default value stored in session.name at request startup time. Thus, you need to call session_name() for every request (and before session_start() or session_register() are called).
name
       The session name references the session id in cookies and URLs. It
       should contain only alphanumeric characters; it should be short and
       descriptive (i.e. for users with enabled cookie warnings).
       If name is specified, the name of the current
       session is changed to its value.
      
| Warning | 
| The session name can't consist of digits only, at least one letter must be present. Otherwise a new session id is generated every time. |