session_set_save_handler is used before session_start.if your session is setted as auto start. it will return FALSE value.so you need add session_write_close() before session_set_save_handler to cancel the session's auto start.it likes this:
<?php
/*
qq:290359552
*/
session_write_close(); //cancel the session's auto start,important
function open()
{
...
}
....
session_set_save_handler( ... );
session_start();
?>