The PHP script
Now that Flash has sent our updated myXML object to the PHP script on the server, we'll have a look at how this script works. As you could see on the previous page, the script is named processXML.php and here is how it looks:

<?php

$xmlString = $HTTP_RAW_POST_DATA;

if (is_null($xmlString)) {

print "No data was sent";

}

else {

$file = fopen("guestbook.xml", "w+") or die("Can't open XML file");

if(!fwrite($file, $xmlString)){

print "Error writing to XML-file";

}

print $xmlString."\n";

fclose($file);

}

?>

Explanation:

1. In the first line, the data that was sent from Flash is retrieved and assigned to the $xmlString variable. Data sent using the XML object is not URL encoded. Therefore PHP won't parse the data into variables and you will need to use $HTTP_RAW_POST_DATA to access the raw data directly. $xmlString now contains one long string where the new guestbook entry is already inserted.
2. The if statement then uses the is_null() function to check if the $xmlString variable really contains a value. If for some reason the data is not retrieved correctly or the PHP script is called directly from the browser, we could at the end of the script end up with an empty XML file.
3. In the else statement, if $xmlString does contain data, we use the fopen() function to open the file guestbook.xml using the w+ mode. This mode opens the file for reading and writing, places the file pointer at the beginning of the file and truncates the file to zero length, meaning that all content is erased. If the file does not exist, it will attempt to create it. The result of the fopen() action is assigned to the $file variable, meaning that $file can now be used as a pointer to our XML file. Note that the file must be accessible to PHP, so you need to ensure that the file permissions allow this access. This means that you must set (CHMOD) permissions to 777, which you can do with your FTP program for example.
4. Within the following if-statement the script attempts to write this 'xml-string' back to our guestbook.xml file.
5. After that the $xmlString variable is sent back to Flash where it is received in our receiverXML object.
6. Finally a bit of cleaning up is done to free up the used file descriptor: our XML file is closed using the fclose() method.

And now we're almost done. By sending our data back to Flash, the receiverXML.onLoad handler is invoked. Within this handler the showXML() prototype is called again, just as it was when the flash movie first loaded. This function again loops through our - now updated - XML file and shows all entries in our textfield so we can read what we've just written in the guestbook

This leaves me with one final note taken from Macromedia's Technotes:

Note

Problem: Loading more than 64k of data using the LoadVars.load, loadVariables, XML.load, or XML.sendAndLoad actions can cause poor browser performance. Common problems can include 501errors, "not implemented" errors, or general browser slowness. Solution: Load the information in smaller packets. Using multiple load actions and spreading the data out over a series of frames can reduce the work the Macromedia Flash Player needs to do to load and parse the information. This can greatly increase browser and loading performance.

After done some testing with an XML file exceeding 500 Kb (over 700 entries) I must say I haven't experienced this problem however.
To wrap up this tutorial: although a mySQL database offers a much more powerful way to create a guestbook, it is actually possible to create a simple one using only Flash, PHP and an XML file.

I hope this tutorial has been useful for you. If you have any questions, feel free to post on the forums... but first download the zipped guestbook and give it a go ;)

Download ZIP

Cheers!