> For the complete documentation index, see [llms.txt](https://php-fhir-tools.ardenexal.net/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://php-fhir-tools.ardenexal.net/models/fhir-versions.md).

# FHIR Versions: R4 / R4B / R5

{% hint style="info" %}
This is the **one place** in the docs where FHIR-version differences are documented. The rest of the library (serialization, FHIRPath, validation) behaves the same across versions — only the model classes differ.
{% endhint %}

Each FHIR version is generated into its own namespace under `Ardenexal\FHIRTools\Component\Models\`:

| Version | Namespace root  | Core package        |
| ------- | --------------- | ------------------- |
| R4      | `…\Models\R4\`  | `hl7.fhir.r4.core`  |
| R4B     | `…\Models\R4B\` | `hl7.fhir.r4b.core` |
| R5      | `…\Models\R5\`  | `hl7.fhir.r5.core`  |

Because the version is a namespace segment, the same resource (e.g. `PatientResource`) exists independently in each version and the classes never clash. Each namespace carries the identical subfolder layout (`Resource`, `DataType`, `Primitive`, `Enum`, `Extension`, `Profile`) described in [Namespace Organization](/models/namespaces.md).

{% tabs %}
{% tab title="R4" %}

```php
use Ardenexal\FHIRTools\Component\Models\R4\Resource\PatientResource;

$patient = new PatientResource(/* … */);
```

{% endtab %}

{% tab title="R4B" %}

```php
use Ardenexal\FHIRTools\Component\Models\R4B\Resource\PatientResource;

$patient = new PatientResource(/* … */);
```

{% endtab %}

{% tab title="R5" %}

```php
use Ardenexal\FHIRTools\Component\Models\R5\Resource\PatientResource;

$patient = new PatientResource(/* … */);
```

{% endtab %}
{% endtabs %}

## Using multiple versions side by side

Because each version lives in a separate namespace, you can import classes from more than one version in the same file. Use PHP namespace aliasing to disambiguate same-named classes:

```php
use Ardenexal\FHIRTools\Component\Models\R4\Resource\PatientResource as R4Patient;
use Ardenexal\FHIRTools\Component\Models\R4B\Resource\PatientResource as R4BPatient;
use Ardenexal\FHIRTools\Component\Models\R5\Resource\PatientResource as R5Patient;

$r4  = new R4Patient(/* … */);
$r5  = new R5Patient(/* … */);
```

{% hint style="info" %}
Models are not committed for every version by default. Generate the versions you need with `composer run generate-models-r4`, `generate-models-r4b`, `generate-models-r5`, or `generate-models-all`. See [Generating Implementation Guides](/code-generation/implementation-guides.md) and the [command reference](/reference/commands.md).
{% endhint %}


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