here is a code i use.
//Name the the variable of $text is what you put in the form field called "totext".
$text = $_POST["totext"]
//Define what is to be sent to the text file
$topost = "<p>$text</p>"
//Send it to the text file.
$fp5 = fopen("textfile.txt", 'w');
the w means to write to the file, this over writes anything in there. to make it add to the bottom, change the w to a.
you have to be careful with php characters also, with things like " and / and '.