将textarea的内容保存到文件时fwrite不会写入

I'm making an online text editor - just a simple one for my own stuff. I'm trying to write code to save the contents of a textarea to a file - first the file is opened into the textarea (which works fine), and then I want to save the edited text. The writing isn't working. Here's the important code (my apologies for the mess, it's very early days for me and PHP):I'm making an online text editor - just a simpl