api/controllers/ApiRouter.php
private function outputTxt($aOut)
    {
        $oOutput = Factory::service('Output');
        $oOutput->set_content_type('text/html');
        if (Environment::not(Environment::ENV_PROD) && defined('JSON_PRETTY_PRINT')) {
            return json_encode($aOut, JSON_PRETTY_PRINT);
        } else {
            return json_encode($aOut);
        }
    }
private function outputTxt($aOut)
    {
        $oOutput = Factory::service('Output');
        $oOutput->set_content_type('text/html');
        if (Environment::not(Environment::ENV_PROD) && defined('JSON_PRETTY_PRINT')) {
            return json_encode($aOut, JSON_PRETTY_PRINT);
        } else {
            return json_encode($aOut);
        }
    }
Formats $aOut as a plain text string formatted as JSON (for easy reading)
but a plaintext contentType

@param  array $aOut The result of the API call

@return string