PHP to csv file?

I have a form when submitted, a PHP script sends all fields to an E-mail address which works great. I would like all these fields going to a csv file everytime the form is submitted. Can the csv file keep growing with new data everytime the form is submitted? Or, can a unique csv file be created each time a form is submitted? If someone can provide a PHP script and a step-by-step on how to set this up I would be eternally grateful.

Thank you,
Jeff