Using gmdate will always return a GMT date. Syntax is same as for date.

A simple gmdate() will suffice

<?php
print gmdate("Y-m-d\TH:i:s\Z");

As previously answered here, since PHP 5.2.0 you can use the DateTime class and specify the UTC timezone with an instance of DateTimeZone.

The DateTime __construct() documentation suggests passing "now" as the first parameter when creating a DateTime instance and specifying a timezone to get the current time.

$date_utc = new \DateTime("now", new \DateTimeZone("UTC"));

echo $date_utc->format(\DateTime::RFC850); # Saturday, 18-Apr-15 03:23:46 UTC

$time = time();
$check = $time+date("Z",$time);
echo strftime("%B %d, %Y @ %H:%M:%S UTC", $check);

Obtaining UTC date

gmdate("Y-m-d H:i:s");

Obtaining UTC timestamp

time();

The result will not be different even you have date_default_timezone_set on your code.

date("Y-m-d H:i:s", time() - date("Z"))

Other than calling gmdate you can also put this code before your rest of the code:

<?php
date_default_timezone_set("UTC");
?>

That will make rest of your date/time related calls to use GMT/UTC timezone.

You can use gmmktime function without arguments to obtain the current UTC timestamp:

$time = gmmktime();
echo date("Y-m-d H:i:s", $time); 

gmmktime will only work if your server time is using UTC. For example, my server is set to US/Pacific. the listed function above echos back Pacific time.

with string GMT/UTC +/-0400 or GMT/UTC +/-1000 based on local timings

Your custom format is just missing O to give you the timezone offsets from local time.

Difference to Greenwich time (GMT) in hours Example: +0200

date_default_timezone_set('America/La_Paz');
echo date('Y-m-d H:i:s O');

2018-01-12 12:10:11 -0400

However, for maximized portability/interoperability, I would recommend using the ISO8601 date format c

date_default_timezone_set('America/La_Paz');
echo date('c');

2018-01-12T12:10:11-04:00

date_default_timezone_set('Australia/Brisbane');
echo date('c');

2018-01-13T02:10:11+10:00

You can use also gmdate and the timezone offset string will always be +00:00

date_default_timezone_set('America/La_Paz');
echo gmdate('c');

2018-01-12T16:10:11+00:00

date_default_timezone_set('Australia/Brisbane');
echo gmdate('c');

2018-01-12T16:10:11+00:00

You can use following to get UTC time:

date_default_timezone_set('Asia/Calcutta');

$current_date = date("Y/m/d g:i A");

$ist_date = DateTime::createFromFormat(
                        '"Y/m/d g:i A"',
                        $current_date,
                        new DateTimeZone('Asia/Calcutta')
                    );

$utc_date = clone $ist_date;
$utc_date->setTimeZone(new DateTimeZone('UTC'));

echo 'UTC:  ' . $utc_date->format('Y-m-d g:i A');

/**
     * Converts a local Unix timestamp to GMT
     *
     * @param   int Unix timestamp
     * @return  int
     */
    function local_to_gmt($time = '')
    {
        if ($time === '')
        {
            $time = time();
        }

        return mktime(
            gmdate('G', $time),
            gmdate('i', $time),
            gmdate('s', $time),
            gmdate('n', $time),
            gmdate('j', $time),
            gmdate('Y', $time)
        );
    }

You'd like to use gmdate method to get GMT date.

gmdate("Y-m-d H:i:s");

gmdate date-formats resemble those in the date() method.

$code->created_at->setTimezone('UTC')

here $code->created_at = Asia/Dhaka Time.

Below find the PHP code to get current UTC(Coordinated Universal Time) time

<?php
// Prints the day
echo gmdate("l") . "<br>";

// Prints the day, date, month, year, time, AM or PM
echo gmdate("l jS \of F Y h:i:s A");
?>