$quotes = file("http://path/to/file.txt");
$quote = rand(0, sizeof($quotes)-1);
each line of code in the txt file is seperated by a ENTER hit
Since it randomly selects a line of code to replace it.you can insert php into the txt file, and it will display it as php.
and you can use it to randomly display text/img banners etc
ps remember that since it displaying the code randomly is the actual code.you need to write "echo" and other code when displaying regular HTML in the txt file
Edit - Fixed center code