PHP cookies in a session handler

Answer 1

From the manual for session_set_save_handler():

Note: The "write" handler is not executed until after the output stream is closed. Thus, output from debugging statements in the "write" handler will never be seen in the browser. If debugging output is necessary, it is suggested that the debug output be written to a file instead.

Essentially, writing changes to session data (calling the registered session write function) does not happen until PHP is almost completely done with its execution cycle. By this time, all your output has already been sent and it is not possible to modify the header information, so setcookie() won't work.

You can have it write data earlier by using session_write_close().