Customized Session Handler can not work for PHP 5.1.6 and CentOS 5.5

Hi, All, I can't see the post I sent several hours ago, if repeated, please reply to this one. Thanks! System: CentOS 5.5; PHP version is 5.1.6. I met a strange problem associate with session_save_handler in current environment(The same code can work well in my local windows platform and ubuntu system). I just want to use a customized session save handler to be triggered, so that I can call my own logic to handling the session. The testing in local is pretty great but when migration to the VPS, it bring me the following error: Fatal error: session_start() [<a href='function.session-start'>function.session-start</a>]: Failed to initialize storage module: user (path: /tmp) The default value for session save handler and session save path in php.ini are: session.save_handler = files session.save_path = "/tmp" session.name = PHPSESSID And the bottom are the code for the session handler. I first called ob_start(), and after calling session::init(), I called session_start(). Then the fatal error happen. It did not trigger any function in the "session" class. I tried change the php.ini from session.save_handler = user, but the error remains. And I found no matter what session.save_handler type is, after calling session_set_save_handler(), the session.save_handler will always automatically changed to 'user', that's why the Fatal error info shows user (path: /tmp). Can anybody help me out for such error? I was stuck by this issue for more than 2 days, but still haven't get any clue! You can view more details from stackoverflow if you want. Here is the link: http://stackoverflow.com/questions/8845924/session-set-save-handler-class-for-database-not-working