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Fusio has a general mechanism to serialize the action response to the fitting format for a client. The serializer is used in case the action returns an array or stdClass. It is also possible to return a string as response body but in this case Fusio simply redirects this response.

Fusio respects also the HTTP Accept header. This means if the HTTP request contains i.e. the header Accept: application/xml Fusio uses the XML serializer to generate a XML response from the data. By default Fusio uses the JSON serializer. Besides the Accept header it is also possible to explicit define a serializer by using the format query parameter i.e. ?format=xml

Custom writer

If you want to develop a custom serializer you can create a class which implements the PSX\Data\WriterInterface. This class receives the raw data and needs to return a string. To add your custom class you need to add the writer to the io service at the container.php file:

/** @var PSX\Data\Processor */
$container->set('io', function($c){
$config = \PSX\Data\Configuration::createDefault(

$phpWriter = new \App\Writer\Php();


return new \PSX\Data\Processor($config);

As example we develop a custom writer which serializes the response data using the native PHP serialize function. In reality this is not really useful since the format is not language independent but it shows the general mechanism:

namespace App\Writer;

class Php implements \PSX\Data\WriterInterface
public function write($data)
return serialize($data);

public function isContentTypeSupported(\PSX\Http\MediaType $contentType)
return $contentType->getName() == 'application/php';

public function getContentType()
return 'application/php';

Also at the configuration.php file you need to add the class to the allowed writer classes:

'psx_supported_writer'    => [

Then it would be possible to use the writer by using the Accept: application/php header or the query parameter ?format=php.