Get friendly with strtotime() and date(); they'll happily work out stuff like that for you.
Monday in the current week can be found by using date() to tell you what day of the week it is now, and that will tell you when Monday was. You can use strtotime() to work out when subsequent days are, or, if Daylight Saving isn't a problem, just add 86400 seconds per day.
Then you can use date() again to get the timestamps into the format you desire.