The && operator (and ||) returns an integer value even when both arguments are boolean. Thus the following code:

function is_true($val)
{
   return (is_bool($val) && $val);
}

echo (is_true(true) ? 'T' : 'F'),"\n";
echo (is_true(is_true(true)) ? 'T' : 'F'),"\n";

gives

T
F

-- Nick