Joomla PHP Download Dialog dynamic image invalid

Question

I'm new to Joomla and PHP dev so bear with me.

Pulling some simple data after a quiz to draw a certificate image. I need users to click on a link and download that image.

Once again, I can get something working outside of Joomla but not within. I haven't even gotten to the dynamically created image yet and running into issues with just a static one, namely the file downloaded is invalid.

Here's one script I was able to get working outside of Joomla, using the template preview as an existing image:

<?php
header("Content-type: image/png");  
header('Content-Disposition: attachment; filename="template_preview.png"');  
readfile('template_preview.png');  

When downloaded the image is correct.

However, in my Joomla file:

<?php defined('_JEXEC') or die;
require_once("includes/variables_certificate.php"); 
header("Content-type: image/png");  
header("Content-Type: application/octet-stream");  
header('Content-Disposition: attachment; filename="template_preview.png"');  
$filename = JPATH_SITE.'/templates/'.$this->template.'/template_preview.png';  
readfile($filename);  

I get an invalid image. This isn't even the part where I'm pulling in the dynamic image yet :/

Any ideas?

The only other thread I could find from Joomla was here and not quite the same issue: Forcing file download in PHP - inside Joomla framework Also followed a bunch of examples in here: http://php.net/manual/en/function.header.php

Thanks all in advance!

#############################################

Partial Solution working for static image (thanks @Chris!), trying to get the dynamic image to work though

download.php:

<?php
require_once("includes/variables_certificate.php");
defined('_JEXEC') or die;
// $filename = JPATH_SITE.'/templates/'.$this->template.'/template_preview.png'; //works
$filename = JURI::root().'index.php?tmpl=certgenerator&quizmod='.$quizmod; //Not working

header('Content-Description: File Transfer');
header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename="certificate.png"');
header('Content-Transfer-Encoding: binary');
header('Expires: 0');
header('Cache-Control: must-revalidate');
header('Pragma: public');
header('Content-Length: ' . filesize($filename));
ob_clean();
flush();
readfile($filename);
exit;

and the certgenerator.php:

defined('_JEXEC') or die;
require_once("includes/variables_certificate.php");

$image_file = JPATH_SITE.'templates/'.$this->template.'/images/certificate_page.png'; 
$my_img = imagecreatefrompng ($image_file);
$font_size_big = 24;
$text_color = imagecolorallocate( $my_img, 0, 255, 0 );
$font_file = JPATH_SITE.'templates/'.$this->template.'/FelipaRegular.ttf';  

imagefttext( $my_img, $font_size_big, 0, 55, 75, $text_color, $font_file, "why u no work"); //if commented out image displays but not font obviously, as it's written now it returns a blank page
imagestring( $my_img, 4, 30, 25, "works", $text_color ); //works but doesn't include font

header( "Content-type: image/png" );
imagepng( $my_img );

imagecolordeallocate( $text_color );
imagedestroy( $my_img );

Answer 1

Use output buffering and flush the file to the browser. Might also want to be sure that defined('_JEXEC') returns true.

<?php
require_once("includes/variables_certificate.php");
defined('_JEXEC') or die;
$filename = JPATH_SITE.'/templates/'.$this->template.'/template_preview.png';

header('Content-Description: File Transfer');
header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename='.basename($filename));
header('Content-Transfer-Encoding: binary');
header('Expires: 0');
header('Cache-Control: must-revalidate');
header('Pragma: public');
header('Content-Length: ' . filesize($filename));
ob_clean();
flush();
readfile($filename);
exit;