The default session.save_path is set to "" which will evaluate to your system’s temp directory. See this comment at https://bugs.php.net/bug.php?id=26757 stating:
The new default for save_path in upcoming releaess (sic) will be the empty string, which causes the temporary directory to be probed.
You can use sys_get_temp_dir to return the directory path used for temporary files
To find the current session save path, you can use
session_save_path()— Get and/or set the current session save path
Refer to this answer to find out what the temp path is when this function returns an empty string.