Editing A Textfile

I am currently writing a script for a course i am doing, It lists all the files and folders on the server, and works similar to windows explorer ( if you click on the folder it shows the content ). I have enabled it so you can delete, view source, and download all files/folders.

Now theres one more thing i would like to do, and that is to enable the user to edit a file, and i am struggling quite a bit, any guidance, ideas or links to a similar tutorial. I am probably going about it completley the wrong way, I was trying to open the file read each line and write it into a textarea, so the user could edit it, and then save it.