> 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/validation/references.md).

# Reference & Target Profile Validation

Validates that a `Reference`- or canonical-typed property points to a resource conforming to at least one of the declared target profile URLs, via `FHIRTargetProfileValidator`, driven by the `#[FHIRTargetProfile]` attribute (which carries a `targetProfiles` list).

{% hint style="info" %}
Reference resolution is pluggable through `FHIRReferenceResolverInterface`. Configure a resolver on the [Configuration](/validation/configuration.md) page. The default `NullFHIRReferenceResolver` skips all target-profile checks silently.
{% endhint %}

## How it works

For each `Reference` object (and each element of an array of references), the validator delegates resolution to the configured `FHIRReferenceResolverInterface::resolve()`:

1. If the resolver returns `null` (including the `NullFHIRReferenceResolver` default), the check is **skipped silently** — no violation.
2. If the resolved object carries no `#[FHIRProfile]` attribute, a WARNING is emitted: `Cannot verify target profile conformance: the resolved object carries no #[FHIRProfile] attribute.`
3. If the resolved object has profiles but none match any URL in `targetProfiles`, an ERROR is emitted listing expected vs. actual profile URLs.

Canonical-typed values (raw `string` or `\Stringable` objects such as `CanonicalPrimitive`) are always skipped — they cannot be resolved to an in-process PHP object. Null values are skipped.

Both messages are overridable via the `FHIRTargetProfile` key in `FHIRValidationMessageRegistry`.

## Resolver examples

See [Configuration](/validation/configuration.md) for wiring a Bundle-scanning or registry-based resolver that maps `Reference` objects to their target PHP model.


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