[9.x] Allow jsonSerialize() to return other types

[mikewuu]

This PR updates jsonSerialize()'s return type to mixed, matching PHP's definition.

Use case

Allows models to override jsonSerialize, and serialize into strings rather than always having to be an JSON object.

Example

We have a simple model, Permission, who's only attribute is name:

class Permission extends Model
{
    protected $fillable = [
        'name',
    ];
}

When we fetch all permissions, the collection is automatically serialized into JSON objects.

Controller

class PermissionsController
{
  public function list()
  {
     return Permission::all();
  }
}

Would currently return

[
  {name: "foo"},
  {name: "bar"},
  {name: "baz"},
]

With this change, Permission, can override jsonSerialize, and return a string.

Updated response after this PR:

["foo", "bar", "baz"]

[driesvints]

Ping @X-Coder264 this is what I meant on that pr your sent in ^

[GrahamCampbell]

This change looks wrong to me. Having an implementation return a more specific type than that required by the interface is totally OK, and actually desirable in this case. https://3v4l.org/CYkJl

[mikewuu]

Updated it to be a union type with string instead. JsonSerializable only specifies that the return type has to be compatible with json_encode, which a string is.

[dennisprudlo]

With array|string it won't be possible to override jsonSerialize and return an integer, a float or a boolean, right? json_encode's $value can have any type but a resource.

[driesvints]

@X-Coder264 my ping was more about you being okay with this change?

[X-Coder264]

@driesvints There's nothing intrinsically wrong with this change. From my perspective though it's a bit weird for a collection (which is just arrays on steroids) to be JSON serialized as anything else but an array. But if people want to do that I guess there's no reason from a framework perspective to block that so on that level I'm fine with the proposed change.

[mikewuu]

@X-Coder264

it's a bit weird for a collection (which is just arrays on steroids) to be JSON serialized as anything else but an array.

It's actually the Collection item's jsonSerialize that I'm hoping to override. The collection can (and should?) still expect an array to be returned.

I've updated it to still return an array for collection in: 464837d

[driesvints]

If we do this then I suggest we move to the mixed return type like the PHP interface and not bother with union types.