You can change the session backend datastore from a script using session_module_name(). 

<?php
  session_module_name("files");  // ASCII files

  session_module_name("mm");     // Shared memory

  session_module_name("user");   // Custom session backend
?>
You can also define your own custom session backend datastore using the session_set_save_handler() function. 

<?php
  session_set_save_handler("myOpen", "myClose",
                         "myRead", "myWrite",
                         "myDestroy", "myGC");
?>
You would then write these 6 functions.