[PHP] Generate a timezone list with optgroups

generate_timezone_list.php

/**
 * Inspired by http://stackoverflow.com/questions/1727077/generating-a-drop-down-list-of-timezones-with-php
 * @return array
 */

function generate_timezone_list()
{
    static $regions = [
        'Africa' => DateTimeZone::AFRICA,
        'Americas' => DateTimeZone::AMERICA,
        // 'Antartica' => DateTimeZone::ANTARCTICA,
        'Asia' => DateTimeZone::ASIA,
        'Atlantic' => DateTimeZone::ATLANTIC,
        'Australia' => DateTimeZone::AUSTRALIA,
        'Europe' => DateTimeZone::EUROPE,
        'Indian Ocean' => DateTimeZone::INDIAN,
        'Pacific' => DateTimeZone::PACIFIC,
    ];

    $region_list = [];

    foreach ($regions as $name => $region) {
        $region_list[$name] = DateTimeZone::listIdentifiers($region);
    }

    $timezone_offsets = [];

    foreach ($region_list as $name => $timezones) {

        foreach ($timezones as $timezone) {
            $tz = new DateTimeZone($timezone);
            $timezone_offsets[$name][$timezone] = $tz->getOffset(new DateTime);
        }

        asort($timezone_offsets[$name]);
    }

    // sort timezone by offset
    $timezone_list = array();
    foreach ($timezone_offsets as $name => $timezones) {
        foreach ($timezones as $timezone => $offset) {
            $offset_prefix = $offset < 0 ? '-' : '+';
            $offset_formatted = gmdate('H:i', abs($offset));
            $pretty_offset = "UTC${offset_prefix}${offset_formatted}";
            $timezone_list[$name][$timezone] = "(${pretty_offset}) $timezone";
        }
    }

    return $timezone_list;
}