Assigning function return value DIRECTLY in template - Smarty 2

I have global function defined in php:

function tabIsProtected($tabNr)
{
    ...
    return true;
}

Now this works in Smarty:

    {if tabIsProtected($tabnr)}
        {assign var="tabProtected" value=true}
    {else}
        {assign var="tabProtected" value=false}
    {/if}

And this is not:

{assign var="tabProtected" value=tabIsProtected($tabnr)}

When I print $tabProtected variable I get string "tabIsProtected(9)" (for $tabnr = 9) and function tabIsProtected isn't even called!

Is there any way to assign function return value directly in template using Smarty 2?

IMPORTANT: I don't want $smarty->assign(...) in php or Smarty3 solutions. Just one liner directly in template.