how to add 2 row data in mpdf

I want to add two rowdata in one value using mpdf 

Code:

<td align="center"><h3>'. $row_dummy['remun'].'</h3></td>
<td align="center"><h3>'. $row_dummy['lum'].'</h3></td>

i try 1

Code:

<td align="center"><h3>'. <?php {$row_dummy['remun']+$row_dummy['lum']} ?> .' </h3></td>

no display