Arduino ethernet webclient php script and mysql

client.println("GET /http://192.168.1.20/var/www/arduino.txt HTTP/1.0");

==>

client.println("GET /arduino.txt HTTP/1.0");

you allready connected to the server, port 80 == HTTP, /var/www is probably the docroot of your webserver...

• HTTP Help: How to test HTTP using Telnet