My sessions work (both file-based and mm-based), but the session module can only be changed in php.ini.

session_module_name([string module]) is supposed to
1. Change the session module if the 
   optional module parameter is specified.
2. Return the name of the current session
   module.

However, it seems that only part two works.

The php.ini (just this line):
session.save_handler=mm

Sample code:

<?php
$name=session_module_name('files');
session_start();

if (!isset($a))
{
	session_register('a');
	$a=0;
}

print "<p>You have reloaded this page $a
	times.<br>Session module name: $name.</p>";

$a++;
?>

Sample output:
You have reloaded this page 3 times.
Session module name: mm.