Looking at single quotes and dots, looks like you are putting together a string for HTML, correct?
In that case this should work:
PHP Code:
$html = '<td align="center"><h3>'.($row_dummy['remun']+$row_dummy['lum']).' </h3></td>';
You can also pre-calculate everything in model or controller and then just echo new $row_dummy['total'] in view, saves moving logic to view files, might be a bit tidier that way.