public function getExtraProperty(string $key)
    {
        if (isset($this->extraProperties[$key])) {
            return $this->extraProperties[$key];
        }
    }
public function getExtraProperty(string $key)
    {
        if (isset($this->extraProperties[$key])) {
            return $this->extraProperties[$key];
        }
    }
public function getExtraProperty ( string $ key ) { if ( isset ( $ this -> extraProperties [ $ key ] ) ) { return $ this -> extraProperties [ $ key ] ; } }
return the value of an extra property

@return mixed
return the value of an extra property