TCPDF get_post_meta outside the loop

Question

I want to be able to export a html page to PDF. I downloaded TCPDF (mainly because it allows me save High resolution images and shrinks the filesize.) The TCPDF folder is in my theme folder.

If a user clicks on a link in a certain page the current page should be saved as a PDF.

The link contains an element to pass the page ID.

<a href="<?php get_bloginfo('template_directory')?>/tcpdf/export/export_pdf.php?id=<?php echo $id; ?>">Click here to PDF this page</a>

In the export_pdf.php file I used $id = $_GET['id'] to retrieve the ID from the URL. With the ID I want to get_post_meta($id); like that but it gives me the following error:

Fatal error: Call to undefined function get_post_meta() in...

I use get_post_meta to retrieve data from outside the loop but I think I'm wrong :-)

This is the code I use in TCPDF to write HTML:

global $post;
$id = $_GET['id'];
$review = get_post_meta($id);
$thispost = get_post( $review );
$content = $thispost->post_content;
$content;

// output the HTML content
$pdf->writeHTML($content, true, false, true, false, '');

I hope somebody can help me with this.

Answer 1

I've got it fixed:
I needed to add the wp-load file that contains all the WP-functions

include('../../../../../wp-load.php');
global $post;
$id = $_GET['id'];
$content_post = get_post($id);
$content = $content_post->post_content;
$content = apply_filters('the_content', $content);
$content = str_replace(']]>', ']]>', $content);

Now the content loads in the PDF