Context is not actually immutable

[SamMousa]

Steps to reproduce
The specification for context: https://github.com/open-telemetry/opentelemetry-specification/tree/v1.44.0/specification/context

Requires a Context instance to be strictly immutable. The current implementation fails this promise.

What is the expected behavior?
Any supported value in Context::with() should either be deeply cloned, or immutable.

What is the actual behavior?

#[\Override]
    public function with(ContextKeyInterface $key, $value): self
    {
        if ($this->get($key) === $value) {
            return $this;
        }

        $self = clone $this;

        if ($key === self::$spanContextKey) {
            $self->span = $value; // @phan-suppress-current-line PhanTypeMismatchPropertyReal

            return $self;
        }

        $id = spl_object_id($key);
        if ($value !== null) {
            $self->context[$id] = $value;
            $self->contextKeys[$id] ??= $key;
        } else {
            unset(
                $self->context[$id],
                $self->contextKeys[$id],
            );
        }

        return $self;
    }

On clone the array is cloned, but the values are copied. Any object typed values are therefore not cloned.
This means that storing a mutable object could lead to unexpected behavior:

$context = Context::getCurrent()
$key = Context::createKey("test");

$mutableValue = new ArrayObject(['message' => 'hello world']);
$context1 = $context->with($key, $mutableValue);

echo $context1->get($key)['message']; // hello world
$mutableValue['message'] = 'bye';

echo $context1->get($key)['message']; // bye

$key2 = Context::createKey("test2");

$context2 = $context->with($key2, 'test123');
echo $context2->get($key)['message']; // bye
/**
 * meanwhile in another Fiber or any other part of the code:
 */
$mutableValue['message'] = 'hello world';
 
echo $context2->get($key)['message']; // hello world

While I believe that case one could be functioning as intended, I believe case 2 is definitely not what people will expect.
I believe this is not technically solvable in PHP.

If we define a Context to be a bag of keys and values, where keys are opaque identifiers (ContextKey), one could argue that we maybe should just not support non-scalar values.

The least we should do is put a comment in ContextInterface to explain the limitations and deviations from the spec (if you consider this a deviation).

Additional context
This is more of a theoretical problem than a practical one at this point.

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[Nevay]

I wouldn't consider this a deviation from the spec; we must be able to store mutable objects (Spans) in the Context. Nonetheless, adding a comment noting that objects stored in the Context may still be modified internally would provide clarity.