This warning was caused by a dot in front of the equals

Fixed version

Easiest way to fix this is remove the . before the =

$class_single_hero = $code->hero_widget($t_uid = $class_single->uid,$t_class = $p2);

Alternate fix

Add the variable before the line, so it has something to append. 

$class_single_hero = "";
$class_single_hero .= $code->hero_widget($t_uid = $class_single->uid,$t_class = $p2);