Start by writing a PHP script which is initiated by a URL something like:
http://www.mydomain.com/floatSwitch?level=1
The PHP script takes the value of level (which is 1 in this example) and writes it to a table in a mySQL database maybe with a time stamp etc.
You can test this simply by using a web browser, that is without using the arduino.
Once you get that working, from the arduino use an HTTP GET command to issue the same URL.