RRule\RRule::rfcString PHP Method

    public function rfcString($include_timezone = true)
    {
        $str = '';
        if ($this->rule['DTSTART']) {
            if (!$include_timezone) {
                $str = sprintf("DTSTART:%s\nRRULE:", $this->dtstart->format('Ymd\\THis'));
            } else {
                $dtstart = clone $this->dtstart;
                $timezone_name = $dtstart->getTimeZone()->getName();
                if (strpos($timezone_name, ':') !== false) {
                    // handle unsupported timezones like "+02:00"
                    // we convert them to UTC to generate a valid string
                    // note: there is possibly other weird timezones out there that we should catch
                    $dtstart->setTimezone(new \DateTimeZone('UTC'));
                    $timezone_name = 'UTC';
                }
                if (in_array($timezone_name, array('UTC', 'GMT', 'Z'))) {
                    $str = sprintf("DTSTART:%s\nRRULE:", $dtstart->format('Ymd\\THis\\Z'));
                } else {
                    $str = sprintf("DTSTART;TZID=%s:%s\nRRULE:", $timezone_name, $dtstart->format('Ymd\\THis'));
                }
            }
        }
        $parts = array();
        foreach ($this->rule as $key => $value) {
            if ($key === 'DTSTART') {
                continue;
            }
            if ($key === 'INTERVAL' && $value == 1) {
                continue;
            }
            if ($key === 'WKST' && $value === 'MO') {
                continue;
            }
            if ($key === 'UNTIL' && $value) {
                if (!$include_timezone) {
                    $tmp = clone $this->until;
                    // put until on the same timezone as DTSTART
                    $tmp->setTimeZone($this->dtstart->getTimezone());
                    $parts[] = 'UNTIL=' . $tmp->format('Ymd\\THis');
                } else {
                    // according to the RFC, UNTIL must be in UTC
                    $tmp = clone $this->until;
                    $tmp->setTimezone(new \DateTimeZone('UTC'));
                    $parts[] = 'UNTIL=' . $tmp->format('Ymd\\THis\\Z');
                }
                continue;
            }
            if ($key === 'FREQ' && $value && !array_key_exists($value, static::$frequencies)) {
                $frequency_key = array_search($value, static::$frequencies);
                if ($frequency_key !== false) {
                    $value = $frequency_key;
                }
            }
            if ($value) {
                if (is_array($value)) {
                    $value = implode(',', $value);
                }
                $parts[] = strtoupper(str_replace(' ', '', "{$key}={$value}"));
            }
        }
        $str .= implode(';', $parts);
        return $str;
    }

 /**
  * @dataProvider rfcStringsGenerated
  */
 public function testRfcStringsGenerated($params, $expected_str)
 {
     $rule = new RRule($params);
     $this->assertEquals($expected_str, $rule->rfcString());
 }