The exception to the principle of "one tainted value taints the whole
expression" is with the ternary conditional operator "?:".  Since code
with a ternary conditional

  $result = $tainted_value ? "Untainted" : "Also untainted";

is effectively

  if ( $tainted_value ) {
    $result = "Untainted";
  } else {
    $result = "Also untainted";
  }

it doesn't make sense for $result to be tainted.