class PDF extends FPDF {

private $number;

function setNumber($number) {

    $this->number = $number;
    //var_dump($this->number);

}

function Header() {

    // Logo
    $this->Image('assets/backend/img/logo/fingerspot-black.png',10,6,30);
    // Arial bold 15
    $this->SetFont('Arial','B',15);
    // Move to the right
    $this->Cell(80);
    // Title
    $this->Cell(30,10,$this->number,1,0,'C');
    // Line break
    $this->Ln(20);

}

// Page footer
function Footer() {

    // Position at 1.5 cm from bottom
    $this->SetY(-15);
    // Arial italic 8
    $this->SetFont('Arial','I',8);
    // Page number
    $this->Cell(0,10,'Page '.$this->PageNo().'/{nb}',0,0,'C');

}

}

public function printSop($id, $rev, $lang) {

    $fpdf = new PDF();
    $fpdf->AddPage();
    $fpdf->setNumber('12345');
    $fpdf->SetFont('Arial','B',16);
    $fpdf->Cell(40,10,'Print Out SOP!');
    $fpdf->Output();
    exit;

}

i want set header for my pdf with number 12345, but always return null in $this->number