how to write to file on server?

Hi,

I have written a simple application with html5 and javascript that reads data from a .csv file.

The user shall be able to change form fields online and save it back to the file.

How do I - in the simplest case - write a variable from js, let's say (var newContent = "Hello World";) to the mydata.csv.

PHP apparently didn't work. Do you support php or which language do you use?

I really just need a quick and dirty approach for a dev-noob.

Thanks

Thomas