Changing a PHP variable "assign var" for theinvoice pdf


I am trying to change a php variable inside the pdf invoice (invoice.tpl)

{assign var=“new_variable” value=0.82*$order_info.display_shipping_cost}

But when I show the new_variable in the pdf,I can only obtain 0.82, I mean, new variable has not the display_shipping_cost*0.82 value

Is it possible to create a new_variable, and place a new value to it??? in this case the 0.82 of $order_info.display_shipping_cost