score board php/Mysql

Question

I have a database which stores the

• username
• score
• timestamp (i use the Unix time() function)

Whenever user does a specific task the score is increased and corresponding time stamp updated.

I want to retrieve the score board with usernames and scores.Ordering the scores with respect to the scores is easy (iam using ORDER BY score desc). but iam stuck when more than one users have same score but different time stamps. How can i work around such a situation.