We need get datas from last week, last month, yesterday and today. We can do it easily in MySQL.

Getting Current Date

SELECT CURDATE();

Result : 2009-06-04

Getting Last Week Datas

SELECT DATE_ADD('2009-06-04', INTERVAL 7 DAY);

Getting Last Month Datas

SELECT DATE_ADD('2009-06-04', 1 MONTH);

Getting Last Year Datas

SELECT DATE_ADD('2009-06-04', INTERVAL 1 YEAR);

And Php example for this calculations, getting last month orders

mysql_query("SELECT * FROM orders WHERE DATE_SUB(CURDATE(),INTERVAL 1 month) <= order_date")); 

For more information of Date and time functions :
http://dev.mysql.com/doc/refman/5.1/en/date-and-time-functions.html