[WORDPRESS,PHP] Sign a pdf with hashed username

functions.php

function signPdf($attachmentId)
{

    // Get the current user
    $currentUser = wp_get_current_user();
    $username = $currentUser->user_login;
    $hashedUsername = hash('sha256', $username);

    // Get the absolute server path of the attachment
    $pdfPath = get_attached_file($attachmentId);

    if (!$pdfPath) {
        // Handle the case where the attachment ID doesn't exist or is not a valid attachment
        return;
    }

    $fontPath = 'calibri-regular.php';

    // Create a new PDF instance
    $pdf = new Fpdi();

    // Get the number of pages in the existing PDF
    $pageCount = $pdf->setSourceFile($pdfPath);

    // Loop through all pages
    for ($pageNumber = 1; $pageNumber <= $pageCount; $pageNumber++) {
        // Add a new page for each existing page
        $pdf->SetAutoPageBreak(false);
        $pdf->AddPage();

        $pdf->AddFont('Calibri', '', $fontPath);
        $pdf->SetFont('Calibri', '', 9);

        // Add text only on the first page
        if ($pageNumber === 1) {
            // Set X-coordinate for right-aligned text
            $xRightAlign = $pdf->GetPageWidth() - 10;

            $pdf->SetXY($xRightAlign, 10);
            $pdf->Cell(0, 0, encodeForPdf('Digitally Signed'), 0, 1, 'R');

            // Adjust Y-coordinate
            $pdf->SetXY($xRightAlign, $pdf->GetY() + 5);

            $pdf->Cell(0, 0, encodeForPdf("Υπογράφοντας"), 0, 1, 'R');

            // Adjust Y-coordinate
            $pdf->SetXY($xRightAlign, $pdf->GetY() + 5);

            $pdf->Cell(0, 0, encodeForPdf($hashedUsername), 0, 1, 'R');

            // Adjust Y-coordinate
            $pdf->SetXY($xRightAlign, $pdf->GetY() + 5);

            // Get date format and time format from WordPress settings
            $dateFormat = get_option('date_format');
            $timeFormat = get_option('time_format');

            $formattedDate = date_i18n($dateFormat . ', ' . $timeFormat);

            $pdf->Cell(0, 0, encodeForPdf($formattedDate), 0, 1, 'R');
        }

        // Import existing PDF page
        $templateId = $pdf->importPage($pageNumber);

        // Use the imported page as a template
        $pdf->useTemplate($templateId, 0, 0);
    }

    // Output the modified PDF to the temporary file
    $pdf->Output($pdfPath, 'F');
}