code: public function assertStringContaining($config, $key, $searchedPart)
      {
          $this->assertString($config, $key);
          if (array_key_exists($key, $config) && strpos($config[$key], $searchedPart) === false) {
              throw new ConfigurationException($key." must contain ".$searchedPart);
          }
          return $this;
      }
docs: Assert a string contains given substring or is not set
  @param array $config Array being validated
  @param string|int $key Inspected array key
  @param string $searchedPart The string being searched
  @return $this
  @throws ConfigurationException On a failed assertation