problema con post-session_start-fpdf

[stabi]

guarda, è banale, questo è il file a cui arrivo dal form in post

<?
//include 'config.inc.php';
//include './include/common_set.php';
$o=ob_start();
$i=session_start();
//include 'funzioni.inc.php';
//$ses=session_id();

define('FPDF_FONTPATH','./fpdf/font/');
require('./fpdf/fpdf.php');


include 'def_stat.php';

?>

questo è il include

<?


//define('FPDF_FONTPATH','fpdf/font/');
//require('fpdf/fpdf.php');


$today=getdate();
$mon = $today['mon'];
$mday = $today['mday'];
$year = $today['year'];
$ora = $today['hours'];
$min = str_pad("".$today['minutes'], 2, "0", STR_PAD_LEFT);


 $when=$mday."/".$mon."/".$year."  Ore ".$ora.":".$min;
//Create new pdf file
  $pdf=new FPDF('L','mm','A4');     // landscape

  $pdf->AliasNbPages();
//Open file
  $pdf->Open();

//Disable automatic page break
  $pdf->SetAutoPageBreak(false);

//Add first page
  $pdf->AddPage();
//set initial y axis position per page
$y_axis_initial =30;

//print column titles for the actual page
// $pdf->SetFillColor(232,232,232);
//$pdf->SetFillColor(300,300,300);

$pdf->SetFont('Arial','',9);
$pdf->SetY($y_axis_initial);
$pdf->SetX(0);
//$y_axis = $y_axis + $row_height;
//initialize counter
$i = 0;

//Set maximum rows per page
$max = 36;
//Set Row Height
$row_height =4.2;
$x_axis_initial = 10;
$x_shift=20;
$x_axis = $x_axis_initial;
$y_axis = $y_axis_initial + $row_height;
$pdf->Cell(100,10,'ciao');
$pdf->Output();
?>

se commento la riga con session_start va' !