DOMPDF Doesn't Work if HTML is Already Present - php

Question

I have a function that uses DOMPDF to generate a .pdf of whatever I set for the $html argument.
function generate_pdf($html){
//DOMPDF stuff
}
This works, but the problem I'm running into is that when I call this function from a page that already has HTML content, it fails.
Fails...
<?php
require_once '../header.php'; //This has HTML content in it.
$html = '<h1>stuff</h1>';
generate_pdf($html);
?>
This also fails...
<?php
echo 'stuff';
$html = '<h1>stuff</h1>';
generate_pdf($html);
?>
Works...
<?php
$html = '<h1>stuff</h1>';
generate_pdf($html);
?>
Is there any way around this?
Contents of function generate_pdf($html)
function generate_pdf($html){
//Get the necessary dompdf files
include_once DOMPDF_PATH . '/autoload.inc.php';
// instantiate and use the dompdf class
$dompdf = new \Dompdf\Dompdf();
$dompdf->loadHtml($html);
// Render the HTML as PDF
$dompdf->render();
//Output the PDF
$dompdf->stream();
}
Note that the file in which this function lives has a namespace, so that's why $dompdf = new \Dompdf\Dompdf(); might appear wrong, but that line is working fine.

Answer 1

CHANGED ANSWER after full code was added to question:
I have the complete HTML content in an included file, no extra HTML before or after it (also no php echoing).
I made a mistake before - I mixed up the inclusion of the dompdf autoload and the content - but see below how it works in my case. This is the content of a php file, there is no function:
<?php
require_once '../../dompdf/autoload.inc.php';
use Dompdf\Dompdf;
$dompdf = new Dompdf();
$dompdf->set_option('isPhpEnabled', true);
ob_start();
//Here I am getting a few variables (from the URL via GET) which I use in the included php file
include_once "your_content_in_one_file.php";
$html = ob_get_contents();
ob_end_clean();
$dompdf->loadHtml($html);
$dompdf->setPaper('A4', 'portrait');
$dompdf->render();
$dompdf->stream($pdf_name);
?>

Answer 2

It ended up being related to the fact that ob_start(); had already been called somewhere else in the system. Using...
if (ob_get_level()){
ob_end_clean();
}
fixed my issue. Hope it helps someone else.