在Drupal中为现有PDF添加水印

I'm working on a project in DRUPAL and am trying to programatically add an image to an existing pdf - an "Approved" image for invoices. I've Frankensteined the code from the StackExchange post - How can I add a watermark to an existing PDF file using PHP? - and have managed to get it working outside of Drupal (7.0) but once I port the code over, I get nothing, no errors, no warnings, nadda .... Just wondering if someone out there can help me on this issue, it been driving me nuts for a bit now .... Sorry about the sloppy code....

<?php
global $user;
ob_start();
$filename ="approved_".$user->name.".png";
$file = "2pages.pdf";
$op = 100;
////////////////////////////////////////////////////////////////////////////
require('fpdf.php');
require('fpdi.php');
$text = "Text message for watermark goes here";
$name = uniqid();
$font_size = 5;
$ts=explode("
",$text);
$width=0;
foreach ($ts as $k=>$string) {
    $width=max($width,strlen($string));
}
$width  = imagefontwidth($font_size)*$width;
$height = imagefontheight($font_size)*count($ts);
$el=imagefontheight($font_size);
$em=imagefontwidth($font_size);
$img = imagecreatetruecolor($width,$height);
// Background color
$bg = imagecolorallocate($img, 255, 255, 255);
imagefilledrectangle($img, 0, 0,$width ,$height , $bg);
// Font color
$color = imagecolorallocate($img, 0, 0, 0);
foreach ($ts as $k=>$string) {
    $len = strlen($string);
    $ypos = 0;
    for($i=0;$i<$len;$i++){
        $xpos = $i * $em;
        $ypos = $k * $el;
        imagechar($img, $font_size, $xpos, $ypos, $string, $color);
        $string = substr($string, 1);      
    }
}
imagecolortransparent($img, $bg);
$blank = imagecreatetruecolor($width, $height);
$tbg = imagecolorallocate($blank, 255, 255, 255);
imagefilledrectangle($blank, 0, 0,$width ,$height , $tbg);
imagecolortransparent($blank, $tbg);

if ( ($op < 0) OR ($op >100) ){
    $op = 100;
}
imagecopymerge($blank, $img, 0, 0, 0, 0, $width, $height, $op);
$pdf = new FPDI();
//********************************************************************
 if (file_exists($file)){
    $pagecount = $pdf->setSourceFile($file);
 } else {
    return FALSE;
 }
 /////////////////////////////////////////
 for($i=1; $i <= $pagecount; $i++) {   
  $tpl = $pdf->importPage($i);    
  $pdf->addPage();     
  $pdf->useTemplate($tpl, 1, 1, 0, 0, TRUE);    
  //Put the watermark  
  $pdf->Image($filename, 0, 0, 40, 25, 'png');}   
  /////////////////////////////////////////
  return $pdf->Output('test10.pdf', 'F');
////////////////////////////////////////////////////////////////////////////
ob_end_flush();
?>

Of note, I am also open you anyone who may have an alternate method to achieve the same end result

thanks in advance!

So after taking some advise from Marc B, I went through the code and isolated the issue. For those looking for the answer to this question, here's the solution I came up with ... Hope it helps!

<?php
ob_start();
function PlaceWatermark($file, $text, $xxx, $yyy, $op, $outdir) {
global $user;
require('fpdf.php');
require('fpdi.php');
$myfilename = 'approved_'.$user->name.'.png'; 

// Created Watermark Image
$pdf = new FPDI();
//if (file_exists("./".$file)){
    $pagecount = $pdf->setSourceFile($file);
//} else {
//    return FALSE;
//}

/////////////////////////////////////////
for($i=1; $i <= $pagecount; $i++) {   
   $tpl = $pdf->importPage($i);    
   $pdf->addPage();     
   $pdf->useTemplate($tpl, 1, 1, 0, 0, TRUE);    
   //Put the watermark  
   if (file_exists($myfilename))  {
             $pdf->Image($myfilename, 0, 0, 40, 25, 'png');
   }  else  {
             $pdf->Image('approved.png', 0, 0, 40, 25, 'png');
   }
 }   
 /////////////////////////////////////////
 return $pdf->Output('test_output.pdf', 'F');

}
PlaceWatermark("Original.pdf", "", 30, 120, 100,TRUE);

ob_end_flush();
?>