如何使用PHP将文本添加到现有PDF中？

I need to add a small text so that it is in the footer of my PDF file. I am trying following code:

    require_once('pdf/fpdf/fpdf.php');
    require_once('pdf/fpdi/Fpdi.php');


    $fullPathToFile = "../assets/files/".$arquivo;
    $pdf = new FPDI();
    $pdf->AddPage(); 

    $pdf->setSourceFile($fullPathToFile); 
    $tplIdx = $this->pdf->importPage(1); 
    $this->pdf->useTemplate($tplIdx, 0, 0, 0, 0, true); 

    $this->pdf->SetFont('Arial', '', '13'); 
    $this->pdf->SetTextColor(0,0,0);
    $this->pdf->SetXY(20, 20);
    $this->pdf->Write(0, 'my text');
    $this->pdf->Output('gift_coupon_generated.pdf', 'D');

but this is returning following error:

Fatal error: Class 'setasign\Fpdi\FpdfTpl' not found in /home/mysite/public_html/painel/pdf/fpdi/Fpdi.php on line 25