1. Warning:  Unknown: open(/tmp\sess_8954767500a61dc446f6f3ec0f768654, O_RDWR) failed: No such file or directory (2) in Unknown on line 0.
2. Warning:  Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/tmp) in Unknown on line 0.

Отправляюсь в php.ini в блок [SESSION]
Вижу:

session.save_handler = files

и

session.save_path = "/tmp"

Читаю тут же комментарий:
; Argument passed to save_handler.  In the case of files, this is the path
; where data files are stored. Note: Windows users have to change this
; variable in order to use PHP's session functions.
;
; As of PHP 4.0.1, you can define the path as:
;
;     session.save_path = "N;/path"
;
; where N is an integer.  Instead of storing all the session files in
; /path, what this will do is use subdirectories N-levels deep, and
; store the session data in those directories.  This is useful if you
; or your OS have problems with lots of files in one directory, and is
; a more efficient layout for servers that handle lots of sessions.
;
; NOTE 1: PHP will not create this directory structure automatically.
;         You can use the script in the ext/session dir for that purpose.
; NOTE 2: See the section on garbage collection below if you choose to
;         use subdirectories for session storage
;
; The file storage module creates files using mode 600 by default.
; You can change that by using
;
;     session.save_path = "N;MODE;/path"
;
; where MODE is the octal representation of the mode. Note that this
; does not overwrite the process's umask.

Соответственно меняю путь до ближайшей мусорки...

session.save_path = "C:\temp"

И ошибки, естессна, исчезли. Но в инете что-то об этом не нашёл.