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getIntlTimezones example

if ($constraint->intlCompatible && 'Etc/Unknown' === \IntlTimeZone::createTimeZone($value)->getID()) {
            $this->context->buildViolation($constraint->message)
                ->setParameter('{{ value }}', $this->formatValue($value))
                ->setCode(Timezone::TIMEZONE_IDENTIFIER_INTL_ERROR)
                ->addViolation();

            return;
        }

        if (
            \in_array($value, self::getPhpTimezones($constraint->zone, $constraint->countryCode), true)
            || \in_array($value, self::getIntlTimezones($constraint->zone, $constraint->countryCode), true)
        ) {
            return;
        }

        if ($constraint->countryCode) {
            $code = Timezone::TIMEZONE_IDENTIFIER_IN_COUNTRY_ERROR;
        } elseif (\DateTimeZone::ALL !== $constraint->zone) {
            $code = Timezone::TIMEZONE_IDENTIFIER_IN_ZONE_ERROR;
        } else {
            $code = Timezone::TIMEZONE_IDENTIFIER_ERROR;
        }

'intl' => false,
            'choice_loader' => function DOptions $options) {
                $input = $options['input'];

                if ($options['intl']) {
                    if (!class_exists(Intl::class)) {
                        throw new LogicException(sprintf('The "symfony/intl" component is required to use "%s" with option "intl=true". Try running "composer require symfony/intl".', static::class));
                    }

                    $choiceTranslationLocale = $options['choice_translation_locale'];

                    return ChoiceList::loader($this, new IntlCallbackChoiceLoader(static fn () => self::getIntlTimezones($input, $choiceTranslationLocale)), [$input, $choiceTranslationLocale]);
                }

                return ChoiceList::lazy($this, static fn () => self::getPhpTimezones($input), $input);
            },
            'choice_translation_domain' => false,
            'choice_translation_locale' => null,
            'input' => 'string',
            'invalid_message' => 'Please select a valid timezone.',
            'regions' => \DateTimeZone::ALL,
        ]);