Saving HTML content generated by PHP as a PDF file

I'm trying to save HTML content generated by PHP as a PDF file.

To do this I found FPDF.

My script is as follows:

if(isset($_POST['content_to_save']) && isset($_POST['name_to_save'])){
    $file_name = $_POST['name_to_save'];
    $file_content = $_POST['content_to_save'];
    require('fpdf.php');
    $pdf = new FPDF();
    $pdf->AddPage();
    $pdf->SetFont('Helvetica','',12);
    $pdf->Cell(40,10,$file_content);
    $content = $pdf->Output('../my_folder/'.$file_name.'.pdf','F');
}

My two variables ($file_name & $file_content) are set as I want them with no issues and it creates the PDF in the correct location with the correct file name, however the actual content is the HTML in text format rather than the actual rendered HTML.

Edit

I have now started trying to use TCPDF

My code now is as follows:

if(isset($_POST['po_content_to_save']) && isset($_POST['po_name_to_save'])){
    $file_name = $_POST['po_name_to_save'];
    $file_content = $_POST['po_content_to_save'];
    require('TCPDF/tcpdf.php');
    $pdf = new TCPDF('P', PDF_UNIT, PDF_PAGE_FORMAT, true, 'UTF-8', false);
    $pdf->SetFont('helvetica', '', 12);  
    $pdf->AddPage();  
    $pdf->writeHTML($file_content);  
    $pdf->Output('../my_folder/'.$file_name.'.pdf', 'I'); 
}

However, now I get the following error:

TCPDF ERROR: Some data has already been output, can't send PDF file

So after a lot of trial and error with a few different libraries I finally found Dompdf.

The following code now does what I was trying to achieve:

require '../vendor/autoload.php';
use Dompdf\Dompdf;

if(isset($_POST['po_content_to_save']) && isset($_POST['po_name_to_save'])){
    $file_name = $_POST['po_name_to_save'];
    $file_content = $_POST['po_content_to_save'];
    
    $document = new Dompdf();
    $document->loadHtml($file_content);
    $document->setPaper('A4', 'portrait');
    $document->render();
    $output = $document->output();
    file_put_contents('../my_folder/'.$file_name.'.pdf', $output);
}