> For the complete documentation index, see [llms.txt](https://quantumphp.gitbook.io/docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://quantumphp.gitbook.io/docs/packages/transformer/contracts.md).

# Contracts

## Transformer interface

Every transformer implements `Quantum\Transformer\Contracts\TransformerInterface`.

```php
interface TransformerInterface
{
    public function transform($item);
}
```

The contract stays deliberately open:

* `$item` may be any value
* the return value may be any value

In most applications, transformers return arrays that are ready for JSON responses or view data.

## Collection transform contract

`Quantum\Transformer\Transformer::transform()` accepts:

* `array $data`
* `TransformerInterface $transformer`

and returns a transformed array.

```php
$result = Transformer::transform($data, $transformer);
```

Behavior that matters in real usage:

* the transformer runs once for each input item
* the same transformer instance handles the full collection
* the returned array keeps input order
* associative keys stay attached to their transformed values
* the callback receives each item value, without array keys

## Type boundaries

With `strict_types=1` and concrete parameter types, PHP enforces the package boundary at the method signature.

That means:

* pass an array as the first argument
* pass an object that implements `TransformerInterface` as the second argument

PHP raises `TypeError` when either argument falls outside that contract.

## Exception and return-value flow

The package forwards the result of each `transform()` call directly into the returned array.

Practical effect:

* exceptions from your transformer move upward unchanged
* mixed return types remain mixed in the final array

This keeps the package predictable and leaves output policy inside your transformer class.

## Stateless package contract

`Transformer::transform()` is static and carries no package-level cache or shared state.

Each call depends on:

* the array you pass in
* the transformer object you pass in


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