Frame 3 is the last keyframe in our counter symbol. It is on this frame that we will display our counter value. To do this we use the text tool and the Text Panel to insert a Dynamic Text field. Name your field "count" and uncheck 'selectable' on the text panel. Set the font, color and size as you wish, and remember to depress the include numbers outline button on the Text Panel.
Now open up the Actions inspector for frame 3 and enter the following: 

count = Number(count) + 1;
loadVariablesNum ("counter.php3?count=" + count, 0);
stop ();

Line one of this code converts count to a number from a string, and adds 1 to its value. We must first convert it to a number because loaded variables begin as strings, (a sequence of literal text), and you can't add a number to a string of text: 1 + "test" is invalid. Line two calls our PHP script, which we will talk about below, and tells it the new value of count.
Line three contains an arbitrary stop() command. Which stops this symbol from looping back to frame 1.

The Script.
PHP is a hyper-text-pre-processor. Basically it allows you to incorporate aspects of logic and programming into previously static HTML in order to generate dynamic HTML pages. It runs on the server and returns plain HTML to the user, so it is compatible with all modern browsers. PHP is much like ASP, but free. More information is available at:
http://www.php.net
We're going to use PHP to rewrite our data file each time our counter is incremented. Open your plain text editor again and enter this code:

<?php
    $fd = fopen("counter.txt", "w");
    fwrite($fd, "count=$count&loaded=1");
    fclose($fd);
?>

Now save your file as "counter.php3", (not counter.php3.txt! Make sure you save it with only the .php3 extension).