The Arduino can not directly read from any file, database, text, or graphics, on the PC. The Arduino can talk to an application that runs on the PC that CAN talk to the database. But, you'll most likely need to write that application.
Spend some time looking at PHP examples on the forum and playground.
PHP can be the wrapper around your mySQL db. An Arduino with ethershield can connect to PHP quite easily .
An Arduino has not enough memory and CPU power to do the encryption for direct MySQL access. And by using PHP you have a clear open interface which you can test in your browser! If you don't have a webserver with PHP, check - http://www.apachefriends.org/en/xampp-windows.html - it includes all you need - can take a few days before you are familiar with all of it...
further you can improve your English by using google translate -> libery => library
Als je toch uit nederland komt ... ...
Ik heb een webserver draaien met alle toeters en bellen erop en eraan.
Echter krijg ik het niet voor elkaar om een variabele uit een bestand.php te halen...
Ik ben al 3 dagen aan het rond neuzen op het forum en de playground, maar heb mijn antwoord nog niet gevonden...
To fetch data from the webserver you use a similar POST - but now only the id of the record you want to extract
<?php
$id = $_POST["id"];Â // the id to fetch
$con = mysql_connect("localhost","name","password");
if (!$con) die('Could not connect: ' . mysql_error());
mysql_select_db("testdb", $con);
$sql = "SELECT data FROM rawdata WHEREÂ id = $id";
$result = mysql_query($sql);
while($row = mysql_fetch_array($result)) echo $row['data'];Â // note this can be 0,1,more rows as result.
mysql_close($con);
echo 'OK';Â Â //
?>