IntlDateFormatter returns incorrect week number for an invalid locale (and doesn't throw an exception either)

[campbell-m]

Description

The following code:

<?php
$date = DateTimeImmutable::createFromFormat('Y-m-d', '2023-09-23', new DateTimeZone('Europe/London'));

foreach (['en', 'xx'] as $locale)
{
    $formatter = new IntlDateFormatter(
        $locale,
        IntlDateFormatter::FULL,
        IntlDateFormatter::FULL,
        null,
        null,
        'w'
    );
    
    echo "Locale: $locale, Week number: " . $formatter->format($date) . "<br>\n";
}

Resulted in this output:

Locale: en, Week number: 38
Locale: xx, Week number: 39

But I expected this output instead:

Locale: en, Week number: 38
Locale: xx, Week number: 38

or else for an exception to be thrown when trying to use 'xx'.

This problem happens on all versions of PHP and all operating systems I have tested.

PHP Version

PHP 8.3.0RC2

Operating System

Windows NT 10.0 build 22621 (Windows 11) AMD64

[mpdude]

IMHO this is a breaking change that should not go into a bugfix release.

There might be multiple reasons why you might be using an invalid locale string, e. g. the locale is being taken from a request parameter, is taken from the LANG environment variable and is set to C or the locale requested is simply not available (generated) on a particular system/installation.

While I agree that all those should be checked/caught by the surrounding/preceding code, turning this into an exception in a bugfix release is not an option IMHO.

Also, I think something that throws an exception should not be added in a RC3 (like 8.3.0RC3), but that's another story.