Sign.php

<?php 

interface Sign
{
  /**
   * @param string $string
   * @return string
   */
  public function __invoke(string $string) : string;
}

SignatureService.php

<?php

class SignatureService implements SignatureServiceInterface
{
  /**
   * @param Sign $sign
   * @param Verify $verify
   */
  public function __construct(Sign $sign, Verify $verify)
  {
    $this->sign   = $sign;
    $this->verify = $verify;
  }
  
  public function sign(string $string) : string
  {
    return ($this->sign)($string);
  }
  
  public function verify(string $signed, string $signature) : bool
  {
    return ($this->verify)($signed, $signature);
  }
}

SignatureValidatorMiddleware.php

<?php

final class SignatureValidatorMiddleware implements MiddlewareInterface
{
  /**
   * @param SignatureServiceInterface $service
   */
  private $service;
  
  public function __construct(SignatureServiceInterface $service)
  {
    $this->service = $service;
  }
  
  public function __invoke(ServerRequestInterface $request, ResponseInterface $response,callable $next = null)
  {
      $this->service->validate($something);
  }
  
 }

Verify.php

<?php 

interface Verify
{
  /**
   * @param string $string
   * @param string $signature
   * @return bool
   */
  public function __invoke(string $signed, string $signature) : bool;
}