I am not aware of any character that does that. It shouldn’t be too hard to split the string where the <b> and </b> occur and to output it as three separate strings turning the bold on and off in between using the FPDF font command.
Unfortunately, that command set is not recognized by FPDF. The “manual” for that extension set (a very poor one) makes no mention of any bold commands that may be contained in a variable. It’s nothing like the PHP ‘echo’ command.