public function addCallback(string $key, callable $callback): self
    {
        if (!isset($this->callbacks[$key])) {
            $this->callbacks[$key] = $callback;
        } else {
            throw new \Exception("Callback with key ($key) already exists");
        }

        return $this;
    }
public function addCallback(string $key, callable $callback): self
    {
        if (!isset($this->callbacks[$key])) {
            $this->callbacks[$key] = $callback;
        } else {
            throw new \Exception("Callback with key ($key) already exists");
        }

        return $this;
    }
public function addCallback ( string $ key , callable $ callback ) : self { if ( ! isset ( $ this -> callbacks [ $ key ] ) ) { $ this -> callbacks [ $ key ] = $ callback ; } else { throw new \ Exception ( "Callback with key ($key) already exists" ) ; } return $ this ; }
Add an individual call back

@param string   $key
@param callable $callback

@throws \Exception

@return MessageVerifier
Add an individual call back