Done!

Create a simple custom API class with PHP


1. Create a base API superclass

class API
{
    protected $response = array();

    public function __construct()
    {
        $this->contentType('application/json');
    }

    protected function contentType($type)
    {
        header("Content-Type: $type");
    }

    protected function responseJson($statusCode, $statusMessage, $data)
    {
        header("HTTP/1.1 $statusCode $statusMessage");

        $this->response['statusCode'] = $statusCode;
        $this->response['statusMessage'] = $statusMessage;
        $this->response['data'] = $data;

        return json_encode($this->response);
    }
}

2. Create a subclass that extends above API

class PasswordServiceAPI extends API
{
    /**
     * Hash the plain password provided
     *
     * http://legacyapp.cpom/api/PasswordServiceAPI/hashPassword?plain=Ab1L853Z24N
     * 
     * @return json
     */
    public function hashPassword()
    {
        if (!isset($_GET['plain']) or empty($_GET['plain'])) {
            echo $this->responseJson(
                422,
                "Unprocessable Entity",
                null
            );

            return;
        }
        
        $hash = bcrypt($_GET['plain']);

        echo $this->responseJson(
                200,
                "Success",
                $hash
        );
    }
}