session_name() returns the name of the current
name is specified, the name of
the current session is changed to its value.
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). 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).
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.
See also the session.name configuration directive.