How to manage timezone for different users of different countries for a php web application?

Question

I am developing web application. The application has different users from different countries.

I have used to manage thier registrations using UTC timestamp. It works fi

Answer 1

/**
 * @function getusertimezonedifference return timezone difference with UTC
 * @param integer $timezonedifference       optional
 * @return string time
 * @Comment Get User TimeZone Difference
 */
public function getusertimezonedifference($timezone = "Asia/Kolkata", $changetoUTC=false, $flg_array=false)
{
    #$timezone = "Asia/Kolkata";+5.30
    #$timezone = "America/Anchorage";-9
    #$timezone = "America/Los_Angeles";-8
    #$timezone = "America/Dawson_Creek";-8
    #$timezone = "America/Chicago";-6
    #$timezone = "America/New_York";-5
    #$timezone = "America/Halifax";-4
    #$timezone = "America/St_Johns";#3.30
    $dayLightFlag = false;
    $system_timezone = date_default_timezone_get();
    $local_timezone = "UTC";
    date_default_timezone_set($local_timezone);
    $local = date("Y-m-d H:i:s");
    date_default_timezone_set("GMT");
    $gmt = date("Y-m-d H:i:s");
    date_default_timezone_set($timezone);
    $required = date("Y-m-d H:i:s");
    date_default_timezone_set($system_timezone);
    $diff1 = (strtotime($gmt) - strtotime($local));
    $diff2 = (strtotime($required) - strtotime($gmt));
    if($changetoUTC) {$diff2 =  (-1*$diff2);}
    // extract hours
    $hours = floor($diff2 / (60 * 60));
    // extract minutes
    $divisor_for_minutes = $diff2 % (60 * 60);
    $minutes = floor($divisor_for_minutes / 60);
    // extract the remaining seconds
    $divisor_for_seconds = $divisor_for_minutes % 60;
    $seconds = ceil($divisor_for_seconds);

    //create string HH:MM:SS
    $ret = $hours.":".(($minutes>9)?$minutes:"0$minutes").":".(($seconds>9)?$seconds:"0$seconds");
    if($flg_array){$ret = array($hours, ((abs($minutes)>9)?$minutes:"0$minutes"), ($seconds>9)?$seconds:"0$seconds");}
    return ($ret);
}