What are the key differences between using the class PDF_HTML and the class PDF in FPDF when creating PDFs in PHP?

The key difference between using the class PDF_HTML and the class PDF in FPDF when creating PDFs in PHP is that PDF_HTML allows for the inclusion of HTML content in the PDF, while the class PDF is used for creating PDFs with manually positioned elements. If you need to include HTML content in your PDF, you should use the PDF_HTML class. If you need more control over the positioning of elements in your PDF, you should use the class PDF.

// Using the PDF_HTML class to create a PDF with HTML content
require('fpdf.php');
require('pdf_html.php');

$pdf = new PDF_HTML();
$pdf->AddPage();
$pdf->SetFont('Arial', '', 12);
$pdf->WriteHTML('<h1>Hello, World!</h1>');
$pdf->Output('example.pdf', 'D');
```

```php
// Using the class PDF to manually position elements in a PDF
require('fpdf.php');

$pdf = new FPDF();
$pdf->AddPage();
$pdf->SetFont('Arial', '', 12);
$pdf->Cell(0, 10, 'Hello, World!', 0, 1);
$pdf->Output('example.pdf', 'D');