Strange behaviour migrating sessions with callbacks from php4 to php5

Question

I have to migrate a php4 app that uses session_set_save_handler() to php5.

In php4 everything was fine, but in php5 the callback functions cannot access the global vars anymore, which were set on the page before session_set_save_handler() was called.

In the example below the global var $g1 cannot be accessed in the session_writer() (which is passed as a callback function)

Is there some explanation for this behavior or can you give a hint on migrating sessions with callbacks from php4 to 5?

This is the pseudo code:

function session_writer($id,$vars) {
  global $g1;
  echo "g1 not defined here: ".is_object($g1);
}

global $g1;
$g1 = SomeObject(); //which is the DB connection for the session writer

session_set_save_handler($o,$c,$r,"session_writer",$d,$g);
session_start();

Answer 1

This is actually noted in the documentation:

As of PHP 5.0.5 the write and close handlers are called after object destruction and therefore cannot use objects or throw exceptions. The object destructors can however use sessions.

It is possible to call session_write_close() from the destructor to solve this chicken and egg problem.

Essentially, you'll have to call session_write_close() from the destructor of your SomeObject, or alternatively, do the following:

<?php register_shutdown_function("session_write_close"); ?>

Either of those solutions should force the writing and closing of the session before all objects are destroyed, allowing you to keep your original callback function.