A Make It Happen to move this into core for CiviCRM is currently running for v4.1. You can help make it happen by contributing here

Discounting events has been requested on the forums a few times. It also popped up both on IRC yesterday and was also needed on civicrm.org for our upcoming UK Training Camp. We decided to see if we could implement this via hooks and minimal / no changes to the core code. Turned out we can implement this this feature using three civicrm hooks and a helper fuction. We improved the core a wee bit by allowing the buildForm hook to inform the tpl system of any form elements added. The tpl then places these form elements at the top of the form. An event can have zero or more coupons. Coupons for an event are stored in the option group / value table. Each coupon code is a string (chosen by admin) and can be used a certain number of times. Coupons can also be used forever. A valid coupon gives the user a percentage discount on the event. Giving the coupon a 100% value basically gives the user a free registration. Coupons can be sent in via the URL or can be entered into the registration form.

Here is the recipe for making it work for an event sign-up page -

• In a drupal module (lets call it civitest), Implement civicrm_buildAmount hook to add a form field to accept the coupon code. As the code checks for coupon value in both get and post request, this step is optional if coupon is being passed by url - e.g http://...civicrm/event/register?id=X&reset=1&discountCode=123456789.

  
  function civitest_civicrm_buildForm( $formName, &$form ) {
      if ( $formName == 'CRM_Event_Form_Registration_Register' &&
           $form->getVar( '_eventId' ) == 3 ) { //use event id here
          $form->addElement( 'text', 'discountCode', ts( 'Discount Code' ) );
  
          // also assign to template
          $template =& CRM_Core_Smarty::singleton( );
          $beginHookFormElements = $template->get_template_vars( 'beginHookFormElements' );
          if ( ! $beginHookFormElements ) { $beginHookFormElements = array( ); }
          $beginHookFormElements[] = 'discountCode';
          $form->assign( 'beginHookFormElements', $beginHookFormElements );
  
          $discountCode = CRM_Utils_Request::retrieve( 'discountCode', 'String', $form, false, null, $_REQUEST );
          if ( $discountCode ) {
              $defaults = array( 'discountCode' => $discountCode );
              $form->setDefaults( $defaults );
          }
      }
  }
  
  

• Implement civicrm_buildAmount hook to apply the discount.

  
  function civitest_civicrm_buildAmount( $pageType, &$form, &$amount ) {
      $eventID = $form->getVar( '_eventId' );
      if ( $pageType != 'event' ||
           $eventID != 3 ) { // use event ID here
          return;
      }
      $discountCode = CRM_Utils_Request::retrieve( 'discountCode', 'String', $form, false, null, $_REQUEST );
      if ( ! $discountCode ) { return; }
  
      list( $discountID, $discountPercent, $discountNumber ) = _civitest_discountHelper( $eventID, $discountCode );
      if ( $discountNumber <= 0 ) { return; }
  
      foreach ( $amount as $amountId => $amountInfo ) {
          $amount[$amountId]['value'] = $amount[$amountId]['value'] -
              ceil($amount[$amountId]['value'] * $discountPercent / 100);
          $amount[$amountId]['label'] = $amount[$amountId]['label'] .
              "\t - with {$discountPercent}% discount";
      }
  }
  

• Implement civicrm_postProcess hook to decrement the coupon usage counter by one. Ah! yes coupon code can be configured to be used for one / finite / infinite number of times :)

      
  function civitest_civicrm_postProcess( $class, &$form ) {
      $eventID = $form->getVar( '_eventId' );
      if ( ! is_a($form, 'CRM_Event_Form_Registration_Confirm') || $eventID != 3 ) { return; }
          
      $discountCode = CRM_Utils_Request::retrieve( 'discountCode', 'String', $form, false, null, $_REQUEST );
      if ( ! $discountCode ) { return; }
  
      list( $discountID, $discountPercent, $discountNumber ) = _civitest_discountHelper( $eventID, $discountCode );
      if ( ! $discountID || $discountNumber <= 0 || $discountNumber == 123456789 ) { return; }
  
      $query = "UPDATE civicrm_option_value v SET v.weight = v.weight - 1 WHERE  v.id = %1 AND v.weight > 0";
      $params = array( 1 => array( $discountID, 'Integer' ) );
      CRM_Core_DAO::executeQuery( $query, $params );
  }
  

• Use following query to add a coupon (say '1234XYZ5678') to your database with a validity of two usage.

  
  INSERT INTO `civicrm_option_group` (`name`, `description`, `is_reserved`, `is_active`) 
  VALUES ('event_discount_3', 'Event Discount', 0, 1); # 3 is the event ID here
  
  SELECT @option_group_id_ed := max(id) from civicrm_option_group where name = 'event_discount_3';
  
  INSERT INTO `civicrm_option_value` (`option_group_id`, `label`, `value`, `name`, `weight`, `is_active`) 
  VALUES (@option_group_id_ed, 'Discount Code', 50, '1234XYZ5678', 2, 1); 
  

  weight=2 implies this coupon can't be used after second usage. value=50 indicates the discount percent. 50% for this coupon. name='1234XYZ5678' is the coupon code.

• Helper function that was used while implmenting the hooks above -

  
  function _civitest_discountHelper( $eventID, $discountCode ) {
      $sql = "
  SELECT v.id as id, v.value as value, v.weight as weight
  FROM   civicrm_option_value v,
         civicrm_option_group g
  WHERE  v.option_group_id = g.id
  AND    v.name = %1
  AND    g.name = %2";
  
      $params = array( 1 => array( $discountCode              , 'String' ),
                       2 => array( "event_discount_{$eventID}", 'String' ) );
      $dao = CRM_Core_DAO::executeQuery( $sql, $params );
      if ( $dao->fetch( ) ) {
          // ensure discountPercent is a valid numeric number <= 100
          if ( $dao->value && is_numeric( $dao->value ) && $dao->value >= 0 &&
               $dao->value <= 100 && is_numeric( $dao->weight ) ) {
              return array( $dao->id, $dao->value, $dao->weight );
          }
      }
      return array( null, null, null );
  }
  

As the whole functionality looks powerful, team is on a discussion to add this feature to core in a future release