I wish to make a small CMS system which I can use to let people who ask me to make a website for them to use when they wish to update their own website.

I just wnat to know how you can have a textarea on a page with the code inside and allow the user to modify it and then a button at the bottom to save. How do you get the specific pages code into the box in the firts place and then how do you save it back in afterwards into the correct location in the code file?

I hope you understand my question.