PHP timezone conversion issue

I have this function which is used to covert request date into GMT00 unix time span and store that time span into database.

When i am trying to convert GMT00 time span to valid GMT timezone like GMT+4:00 then below function return wrong time span

/**
     * Convert date time from one timezone to another timezone
     *
     * @param $datetime (string): Date Time value which needs to be converted
     * @param $is_timestamp (boolean): If $datetime is a timestamp then true, otherwise false
     * @param $stimezone (string): Timezone from which to convert date and time
     * @param $dtimezone (string): Timezone in which to convert date and time
     * @param $format (string): Format in which you need date and time
     *
     * @return (misc) Converted date time string, null on failure
     */

public static function convertDateTime($datetime, $is_timestamp = false, $stimezone = "GMT+00:00", $dtimezone = "GMT+00:00", $format = null) {
    if ($is_timestamp) {
        $datetime = date("Y-m-d H:i:s", $datetime);

    } else {
        $datetime = date("Y-m-d H:i:s", strtotime($datetime));
    }

    try {
        $date = new \DateTime($datetime, new \DateTimeZone($stimezone));
        $date->setTimezone(new \DateTimeZone($dtimezone));

        if (!empty($format)) {
            //return $date->format($format);
            return gmdate($format, $date->getTimestamp());

        } else {
            return $date->getTimestamp();
        }

    } catch (\Exception $e) {
        return null;
    }
}