It seems that if you start a session and register a 3d array with it, that array can be modified once, from having nothing in it to having whatever you put in it the first time.  After that, any attempts to modify it don't stick.  The first modification's results don't update.

Example.

*snip*

session_start();

$array[0][0][0] = 0;
session_register($array);

// If these variables below are called from the same place the session is registered, they'll stick.
$array[1][2][3] = 1;
$array[2][3][1] = 3;

// Now let's assume these are set from a different page
$array[4][4][4] = 2;
$array[4][4][5] = 2;

*snip*

The array only contains [1][2][3] and [2][3][1].

The only compiled support is mysql.  I've got it compiled into apache 1.3.11.