Incorrect `ReflectionClass::getNamespaceName` for anonymous class

Description

The following code:

<?php

namespace Foo {
    interface InterfaceFoo {}
}

namespace Bar {
    new class implements \Foo\InterfaceFoo {};
}

namespace {
    foreach (get_declared_classes() as $class) {
        if (!str_contains($class, '@anonymous')) {
            continue;
        }
        echo 'Class: ', $class, PHP_EOL;
        echo 'Namespace: ', (new ReflectionClass($class))->getNamespaceName(), PHP_EOL;
    }
}

Resulted in this output:

Class: Foo\InterfaceFoo@anonymousD:\a\php-anonymous-class-namespace\php-anonymous-class-namespace\test.php:8$0
Namespace: Foo\InterfaceFoo@anonymousD:\a\php-anonymous-class-namespace\php-anonymous-class-namespace

at Windows and

Class: Foo\InterfaceFoo@anonymous/home/runner/work/php-anonymous-class-namespace/php-anonymous-class-namespace/test.php:8$0
Namespace: Foo

at Ubuntu

But I expected this output instead:

Class: Foo\InterfaceFoo@anonymousD:\a\php-anonymous-class-namespace\php-anonymous-class-namespace\test.php:8$0
Namespace: Bar

Reproduced at https://github.com/werlos/php-anonymous-class-namespace (see the only action there).

PHP Version

8.3.0

Operating System

No response

Start with the supplied PHP reproducer and the ReflectionClass::getNamespaceName() entry point. Compare its output on Windows and Ubuntu with the expected namespace, then inspect linked pull request #12907 before making changes. Done means the anonymous class reports Bar consistently.