when you are displaying the information on your php site, you could:
find the number of days inbetween the posted date and the current date
if the number of days is under 7, then say "this week"
*if the number is over 7, divide it by 7 days in a week to get the approximate number of weeks.
-if the number of weeks is greater than say 6 ("about 6 weeks"), divide the number of weeks by 4 to get the approximate number of months.. and on and on for years and decades etc..
make sense?