Answered $_SESSION not updating on FormIt

[[!formPromoCode]]

if (!isset($_SESSION['promo_code'])) {

  $output = $modx->getChunk('formPromoCode');

  return $output;

} else {
  $output = 'You\'re using Promo Code: ' . $_SESSION['promo_code'];

  return $output;
}

[[!FormIt?
  &customValidators=`validateCoupon`
  &validate=`promo_code:required:minLength=^6^:validateCoupon`
  &promo_code.vTextRequired=`Please enter a Promo Code.`
  &promo_code.vTextMinLength=`Promo Code must have at least 6 characters in length.`
  &submitVar=`promo-code-submit`
]]

<form id="coupon-form" action="[[~[[*id]]]]" method="POST">
  <!-- [[!+fi.validation_error_message:notempty=`<span class="payment-errors">[[!+fi.validation_error_message]] </span>`]] -->
  <input type="hidden" name="nospam:blank" value="[[+fi.nospam]]" />
  <div class="row">
    <div class="large-12 columns">
      <div class="row collapse">
        [[!+fi.error.promo_code:notempty=`[[!+fi.error.promo_code]]`]]
        <div class="small-8 columns">
          <input type="text" name="promo_code" value="[[!+fi.coupon_code]]" />
        </div>
        <div class="small-4 columns"><input class="button postfix" type="submit" name="promo-code-submit" value="Apply" /></div>
      </div>
    </div>
  </div>
</form>

if($value != '123456'){
   $validator->addError($key,'Promo Code is invalid.');
   //$validator->addError($key, $validator->_getErrorMessage($key, 'vTextAllowedFileEmpty', 'No file data found. Please try again!'));
   return false;
}

$_SESSION['promo_code'] = $value;
$_SESSION['success'] = 'Success! Your promo code ('.$value.') has been applied.';
return true;