So you want to build the following "pipeline":
sensors --> Arduino --> ethernetshield --> PHPform --> MYSQL DB --> presentation --> user
A good place to start us to add a LCD to the Arduino to show locally the values of the sensors. That is
sensors --> Arduino --> LCD
if that works start with designing the database (be prepared for multiple Arduino's) and the presentation of the data
Then Add the PHP form to fill the database, initially manually.
Then add the ethernetshield to fill the phpform automatically.
An alternative is to look at pachube as this has automated the back end ...