## join 方法会将集合中的值用字符串连接： ``` collect(['a', 'b', 'c'])->join(', '); // 'a, b, c' collect(['a', 'b', 'c'])->join(', ', ', and '); // 'a, b, and c' collect(['a', 'b'])->join(', ', ' and '); // 'a and b' collect(['a'])->join(', ', ' and '); // 'a' collect([])->join(', ', ' and '); // '' ```