code: public static function get(?string $key = null)
  	{
  		self::init();
  		
  		if ($key === null) {
  			return self::$utmCookie;
  		} else {
  			if (mb_strpos($key, 'utm_') !== 0) {
  				$key = 'utm_' . $key;
  			}
  			if (false === array_key_exists($key, self::$utmCookie)) {
  				throw new UnexpectedValueException(sprintf('Argument $key has unexpecte value "%s". Utm value with key "%s" does not exists.', $key, $key));
  			} else {
  				return self::$utmCookie[$key];
  			}
  		}
  	}
docs: Get all utm values or just value of utm with specific key.
  
  @param string|null $key Default null (return all values as array).
  
  @return string|array|null Return string value, array or null if not set.