Please help: How to get bold and italics in the generated PDF file

When I specify bold or italics, nothing shows up after the <b> tag! For example, this text:
the quick brown <b>fox jumps over</i> the lazy dog
produces just
the quick brown
in the generated pdf file.

The code is here:
<?PHP

$phrase = 'the quick brown fox jumps over the lazy dog<br />'.
'the quick brown <b>fox jumps</b> over the lazy dog';
// ---------------------------------------------------------------

require_once('../html2pdf/html2pdf.class.php');
$html2pdf = new HTML2PDF('P', 'A4', 'en');
$html2pdf->writeHTML($phrase);
$html2pdf->Output('first_PDF_file.pdf', 'F'); //write to a file

echo $phrase;
?>