FPDF - Inline bold text

You could rewrite a part of the extension but it may be easier for you to use the extension writeHTML, then draw a cell (empty text) with a border over the cell you created with writeHTML. By adjusting your cells properly it should work.

Don't forget to use SetY then SetX to position your cells.

Example:

if(isset($_POST['optout']) && $_POST['optout'] == "yes"){
   $pdf->Ln(5);
   $pdf->SetFont('Arial','I',12);
   $pdf->SetTextColor(128);

   //Your text cell
   $pdf->SetY($pos_Y);
   $pdf->SetX($pos_X);
   $pdf->writeHTML('This is my disclaimer. <b>THESE WORDS NEED TO BE BOLD.</b> These words do not need to be bold.');

   //Your bordered cell
   $pdf->SetY($pos_Y);
   $pdf->SetX($pos_X);
   $pdf->Cell($width, $height, '', 1, 0, 'C');
 } 

Use Write() & Rect().

Example:

$pdf->Rect($pdf->GetX(),$pdf->GetY(),2,0.1);
$pdf->SetFont('Arial','',8);
$pdf->Write(0.1,"this is not bold, but this ");
$pdf->SetFont('Arial','B',8);
$pdf->Write(0.1,"is bold.");
$pdf->SetFont('Arial','',8);
$pdf->Ln();

You need to play around with width & height of the Rect() parameters. In this case, I set width = 2 and height to 0.1 User Units.

I find other soluction.

http://fpdf.de/downloads/add-ons/tag-based-formatting.html

copy and paste fragment code write_tag inside file fpdf.php, call function write_tag while was create of pdf, whit this I can make format at the text in pdf.

Here's how I solved it:

$pdf->SetFont('Arial','',10);
$cell = 'This is my disclaimer.';
$pdf->Cell($pdf->GetStringWidth($cell),3,$cell, 0, 'L');
$pdf->SetFont('Arial','B',10);
$boldCell = "THESE WORDS NEED TO BE BOLD.";
$pdf->Cell($pdf->GetStringWidth($boldCell),3,$boldCell, 0, 'L');
$pdf->SetFont('Arial','',10);
$cell = 'These words do not need to be bold.';
$pdf->Cell($pdf->GetStringWidth($cell),3,$cell, 0, 'L');

Essentially, make one cell, change font, then another cell with the width of the text you want to bold, and so on.

There seem to be better tools out there for making PDFs that use HTML / Blade templates and such, though, so you might want to consider using that.