How to bold a variable?

How can I have text within a php variable be bold??

function show_balance_header ($balance, $currency)
{
    (string)$display_output = null;
    $display_output = fees_main::display_amount(abs($balance), $currency, true);
    return $display_output;
}

I want to bold $balance. Is there an easy way or do I need to edit display_amount?

I’ve tried doing this:

"<b>".$balance"</b>" but this did not get the correct variable,

thanks in advance!!

Answer

answer is to edit the inner function display_amount using Raeki’s suggestion, for future questions, i think you must edit the inner-most function as only editing function show_balance_header did not get the correct variable