AliasNbPages

class PDF extends FPDF
{
    function Footer()
    {
        // Go to 1.5 cm from bottom
        $this->SetY(-15);
        // Select Arial italic 8
        $this->SetFont('Arial', 'I', 8);
        // Print current and total page numbers
        $this->Cell(0, 10, 'Page '.$this->PageNo().'/{nb}', 0, 0, 'C');
    }
}

$pdf = new PDF();
$pdf->AliasNbPages();