- timezone_name_from_abbr: documentation ( source)
<?php
$timezoneOffset = "+05:30";
echo $timezone_name = timezone_name_from_abbr("",$timezoneOffset*3600,0);
$timezone = 'Asia/Kolkata';
$time = new DateTime('now', new DateTimeZone($timezone));
$timezoneOffset = $time->format('P');
echo $timezoneOffset;