app\models\Invoice::getRecurrenceRule PHP Method

    private function getRecurrenceRule()
    {
        $rule = '';
        switch ($this->frequency_id) {
            case FREQUENCY_WEEKLY:
                $rule = 'FREQ=WEEKLY;';
                break;
            case FREQUENCY_TWO_WEEKS:
                $rule = 'FREQ=WEEKLY;INTERVAL=2;';
                break;
            case FREQUENCY_FOUR_WEEKS:
                $rule = 'FREQ=WEEKLY;INTERVAL=4;';
                break;
            case FREQUENCY_MONTHLY:
                $rule = 'FREQ=MONTHLY;';
                break;
            case FREQUENCY_THREE_MONTHS:
                $rule = 'FREQ=MONTHLY;INTERVAL=3;';
                break;
            case FREQUENCY_SIX_MONTHS:
                $rule = 'FREQ=MONTHLY;INTERVAL=6;';
                break;
            case FREQUENCY_ANNUALLY:
                $rule = 'FREQ=YEARLY;';
                break;
        }
        if ($this->end_date) {
            $rule .= 'UNTIL=' . $this->getOriginal('end_date');
        }
        return $rule;
    }