I wont work .. only writing on 1st page

Thank you for your question. We just go through your code and found that you set the code only for first page. Kindly check importpages function which takes page number as first argument. You need to create loop for other pages as well. Kindly check the below code for more help.

require_once('FPDI/fpdf/fpdf.php');
require_once('FPDI/fpdi.php');
require_once('FPDI/fpdf_tpl.php');

// Original file with multiple pages 
$fullPathToFile = "sample.pdf";

class PDF extends FPDI {

    var $_tplIdx;

    function Header() {

        global $fullPathToFile;

        if (is_null($this->_tplIdx)) {

            // THIS IS WHERE YOU GET THE NUMBER OF PAGES
            $this->numPages = $this->setSourceFile($fullPathToFile);
            $this->_tplIdx = $this->importPage(1);

        }
        $this->useTemplate($this->_tplIdx, 0, 0,200);

    }

    function Footer() {}

}

// initiate PDF
$pdf = new PDF();

// add a page
$pdf->AddPage();


// The new content
$pdf->SetFont("helvetica", "B", 25);
$pdf->SetTextColor(255, 0, 0);
$pdf->Text(40,120,"Sample Text over overlay");

// THIS PUTS THE REMAINDER OF THE PAGES IN
if($pdf->numPages>1) {
    for($i=2;$i<=$pdf->numPages;$i++) {
        //$pdf->endPage();
        $pdf->_tplIdx = $pdf->importPage($i);
        $pdf->AddPage();
    }
}

//show the PDF in page
//$pdf->Output();

// or Output the file as forced download
$pdf->Output("sampleUpdated.pdf", 'D');