User Sessions Facepalm

http://us2.php.net/session_set_save_handler

  "[For all callback functions] Return value is TRUE for success, FALSE for failure."
  
In ext/session/mod_user.c:

  PS_FUNC(user) {
    /* blah blah */
    zval *retval = ps_call_handler(PSF(func), argc, argc);
    if (retval) {
       convert_to_long(retval);
       return Z_LVAL_P(retval);
    }
    return FAILURE;
  }

In Zend/zend.h:

  typedef enum {
    SUCCESS =  0,
    FAILURE = -1,
  } ZEND_RESULT_CODE;

So say you return false from that callback function.  (int)false => 0, return 0 => SUCCESS!
Similiarly, return true; => return (int)true => 1, return 1 => neither SUCCESS nor FAILURE!