Description:
------------
write(string $sessionId, string $data)
The write callback is called when the session needs to be saved and closed. This 
callback receives the current session ID a serialized version the $_SESSION 
superglobal.

When the variable $_SESSION hasn't got a value the value returned is string(0).

Test script:
---------------
<?php
class Handler
{
	public function write( $SessionID, $sData )
	{
		var_dump( serialize( $_SESSION ) ); // Real $sData Value
		var_dump( $sData ); // Return string(0)...
	}
	public function open( $sSavePath, $sSessionName ){return true;}
	public function close(){}
	public function read( $SessionID ){}
	public function destroy( $SessionID ){}
	public function gc( $iMaxLifeTime ){return true;}
}

$Handler = new Handler();
session_set_save_handler( array( $Handler, 'open' ), array( $Handler, 'close' ), array( $Handler,
		'read' ), array( $Handler, 'write' ), array( $Handler, 'destroy' ), array( $Handler, 'gc' ) );
session_start();
session_write_close();
?>

Expected result:
----------------
string(6) "a:0:{}" string(6) "a:0:{}"

Actual result:
--------------
string(6) "a:0:{}" string(0) ""