Query unknown data structure in GraphQL

I just started to work with GraphQL and I am setting up a server with webonyx/graphql-php at the moment. Since a GraphQL query already has to contain the resulting data structure, I am not quite sure how to get dynamic data. Assumed that I query the content which consists different element types and my final structure should look like this:

{
    "data": {
        "dataset": {
            "uuid": "abc...",
            "insertDate": "2018-05-04T12:12:12Z",
            // other metadata
            "content": [
                {
                    "type": "headline",
                    "text": "I am a headline"
                },
                {
                    "type": "image",
                    "src": "http://...",
                    "alt": "I am an image"
                },
                {
                    "type": "review",
                    "rating": 3,
                    "comment": "I am a review"
                },
                {
                    "type": "headline",
                    "text": "I am another headline"
                }
                // other content elements
            ]
        }
    }
}

How could I write a query for this example?

{
    dataset {
        uuid
        insertDate
        content {
            ????
        }
    }
}

And how would a type definition for the content section look like? There is a defined set of element types (headline, image, review, many more) but their order and number of elements is unknown and they have only one field, type, in common. While writing the query in my frontend, I don't know anything about the content structure. And what would the graphql-php type definition for the content section look like? I couldn't find any similar example online, so I am not sure if it is even possible to use GraphQL for this use case. As an extra information, I always want to query the whole content section, not a single element or field, always everything.

When you're returning an array of Object types, but each individual item could be one of any number of different Object types, you can use either an Interface or a Union. We can use an Interface here since all the implementing types share a field (type).

use GraphQL\Type\Definition\InterfaceType;
use GraphQL\Type\Definition\Type;

$content = new InterfaceType([
    'name' => 'Content',
    'description' => 'Available content',
    'fields' => [
        'type' => [
            'type' => Type::nonNull(Type::string()),
            'description' => 'The type of content',
        ]
    ],
    'resolveType' => function ($value) {
        if ($value->type === 'headline') {
            return MyTypes::headline();            
        } elseif ($value->type === 'image') {
            return MyTypes::image();
        } # and so on
    }
]);

Types that implement the Interface need to do so explicitly in their definition:

$headline = new ObjectType([
    # other properties 
    'interfaces' => [
        $content
    ]
]);

Now if you change the type of the content field to a List of content, you can query only fields specific to each implementing type by using inline fragments:

query GetDataset {
  dataset {
    uuid
    insertDate
    content {
      type # this field is shared, so it doesn't need an inline fragment
      ... on Headline {
        text
      }
      ... on Image {
        src
        alt
      }
      # and so on
    }
  }
}

Please see the docs for more details.