timezone_list_better.php

<?php

if (!function_exists('timezone_list')) {
    function timezone_list()
    {
        static $regions = [
            DateTimeZone::AFRICA,
            DateTimeZone::AMERICA,
            DateTimeZone::ANTARCTICA,
            DateTimeZone::ASIA,
            DateTimeZone::ATLANTIC,
            DateTimeZone::AUSTRALIA,
            DateTimeZone::EUROPE,
            DateTimeZone::INDIAN,
            DateTimeZone::PACIFIC,
        ];

        return collect($regions)->flatMap(function ($region) {
            return DateTimeZone::listIdentifiers($region);
        })->map(function ($timezone) {
            $zone = new DateTimeZone($timezone);
            return [$timezone, $zone->getOffset(new DateTime)];
        })
        ->toAssoc()
        ->ksort()
        ->map(function ($offset, $timezone) {
            $prefix = $offset < 0 ? '-' : '+';
            $formatted = gmdate('H:i', abs($offset));
            return "(UTC{$prefix}{$formatted}) {$timezone}";
        });
    }
}

timezone_list_original.php

<?php

if (!function_exists('timezone_list')) {
    function timezone_list()
    {
        static $regions = [
            DateTimeZone::AFRICA,
            DateTimeZone::AMERICA,
            DateTimeZone::ANTARCTICA,
            DateTimeZone::ASIA,
            DateTimeZone::ATLANTIC,
            DateTimeZone::AUSTRALIA,
            DateTimeZone::EUROPE,
            DateTimeZone::INDIAN,
            DateTimeZone::PACIFIC,
        ];

        $timezones = [];

        foreach ($regions as $region) {
            $timezones = array_merge($timezones, DateTimeZone::listIdentifiers($region));
        }

        $offsets = [];

        foreach ($timezones as $timezone) {
            $zone = new DateTimeZone($timezone);
            $offsets[$timezone] = $zone->getOffset(new DateTime);
        }

        ksort($offsets);

        $timezone_list = [];

        foreach ($offsets as $timezone => $offset) {
            $offset_prefix = $offset < 0 ? '-' : '+';
            $offset_formatted = gmdate('H:i', abs($offset));
            $pretty_offset = "UTC${offset_prefix}${offset_formatted}";

            $timezone_list[$timezone] = "(${pretty_offset}) $timezone";
        }

        return $timezone_list;
    }
}