How to Add a Watermark to an Existing PDF File Using PHP

Watermarking on PDF files using PHP

I'd check out this PHP Class on Github by binarystash. It seems to do what you are asking for.
https://github.com/binarystash/PDFWatermarker

Hopefully this helps.

Add background watermark to pdf files via PHP for existing .pdf files?

Yes you can with FPDF and this add-on http://www.fpdf.de/downloads/addons/9/

How do I add a watermark to a PDF using PHP and without removing dynamic content?

Here's the solution I've came up with:

Editing a pdf with php a cost effective manner isn't viable. If you're working with a single pdf, use an online service to convert it into an HTML file (there are many), then use php to edit in a watermark for that file document. Once that's done, use a service like Prince HTML to convert that HTML file back into a newly watermarked pdf to download.

Applying watermarks on pdf files when users try to download the files

Although there are several very good PDF libs for PHP, if I were writing such a program I'd just shell out to run pdftk but you' still need to generate your watermark.

$tempfile=tempnam();
system("pdftk input_file.pdf background watermark.pdf output $tempfile dont_ask", $errcode);
if (!$errcode && $ih=fopen($tempfile, 'r')) {
    header('Content-Type: application/pdf');
    fpassthru($ih);
    fclose($ih);
} else {
    print "Whoops";
}
unlink($tempfile);

Unable to add watermark image in existing PDF

I have created a library which define all header() , footer() function. I have made changes in the library file.I have remove all watermark generation code from library and call it where the actual PDF is generation function and my error was solved

// initiate PDF library

$pdf = new Digilib();

$pdf->setDate($date);
$pdf->name = ucfirst($name); 

$url_curl = $distination_folder . $Filename;
$fileContent = file_get_contents($url_curl, false, stream_context_create(array('ssl' => array('verify_peer' => false, 'verify_peer_name' => false))));
$pageCount = $pdf->setSourceFile(StreamReader::createByString($fileContent));

for ($pageNo = 1; $pageNo <= $pageCount; $pageNo++) {
  $templateId = $pdf->importPage($pageNo);

  $size = $pdf->getTemplateSize($templateid);

  $pdf->AddPage('P', array($size['width'], $size['height']));

  $pdf->useTemplate($templateId);

  $pdf->Image($logopath, 10, 10, 15, '', 'PNG', '', 'T', false, 300, '', false, false, 0, false, false, false);

  $pdf->SetFont('freeserif', 'b', 15);

  $pdf->Cell(0, 2, $CustomHeaderText, 0, false, 'C', 0, '', 0, false, 'T', 'C');

  $pdf->SetFont('freeserif', 'b', 9);
  
  $pdf->Cell(0, 2, '' . $pdf->getAliasNumPage() . '/' . $pdf->getAliasNbPages(), 0, false, 'C', 0, '', 0, false, 'T', 'M');

  $ImageW = 105; //WaterMark Size
  $ImageH = 80;
  $pdf->setPage($pageNo); //WaterMark Page
  $myPageWidth = $pdf->getPageWidth();
  $myPageHeight = $pdf->getPageHeight();
  $myX = ($myPageWidth / 2) - 50;  //WaterMark Positioning
  $myY = ($myPageHeight / 2) - 40;

  $pdf->SetAlpha(0.35);
  $pdf->Image($watermarkpath, $myX, $myY, $ImageW, $ImageH, '', '', 'C', true, 300);
  $pdf->SetAlpha(1);
  $pdf->SetFooterMargin(0);
}