Our client wants us to implement change history for website articles. What is the best way to do it?
There is a wide variety of ways to do this as you alluded by tagging php, .net, python, and ruby.  You missed a few off the top of my head perl and jsp.  Each of these have their plusses and minuses and is really a question of what best suits your needs.
PHP is probably the fastest reward for time spent.
Ruby, i'm assuming Ruby on Rails, is the automatic Buzz Word Bingo for the day.
.Net, are you all microsoft every where and want easy integration with your exchange server and a nice outlook API?
python? Do you like the scripted languages but you're too good for php and ruby.
Each of these languages have their strong points and their draw backs and it's really a matter of what you know, how much you have to spend, and what is your timeframe.
What is the best way to store change history of website articles?