How can I write text to an external file using JQuery/JavaScript/PHP?

I have some text stored in a POST variable that I need to append to an HTML file. I want to do within my PHP webpage, but it could use HTML, JQuery, Javascript, or anything else that will run natively. I've tried the PHP approach, but it hasn't worked despite my numerous attempts. Always a 500 internal server error. And I can't find anything on writing to files using JQuery, so if anyone out there knows something I don't, it would be appreciated. Thanks :)