File_pdf problem

I tried to redefine the functions header() and footer() in a simply new class but my new class print empty header and footer.
Are there any options to set ?
excuse for my bad english.

ermes

my code
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class stampe_quest extends File_PDF {

private $db;
private $pdf;

function __construct() {
$this -> pdf = &File_PDF::factory(array('orientation' => 'P', 'format' => 'A4'));
$this -> pdf -> open();
$this -> pdf -> setCompression(true);
$this -> pdf -> addPage();
...
...
$this -> pdf -> setDisplayMode('fullpage','single');
$this -> pdf -> close();
$this -> pdf -> output("xxxxx.pdf", false);
}

function header() {
// Select Arial bold 15
$this->setFont('Arial', 'B', 15);
// Move to the right
$this->cell(80);
// Framed title
$this->cell(30, 10, 'Title', 1, 0, 'C');
// Line break
$this->newLine(20);
}

function footer() {
// Go to 1.5 cm from bottom
$this->setY(-15);
// Select Arial italic 8
$this->setFont('Arial', 'I', 8);
// Print centered page number
$this->cell(0, 10, 'Page ' . $this->getPageNo(), 0, 0, 'C');
}
}