A systemvariable is used to help us with identifying a row. These systemvariables are variables generated and maintained by the mSQL server. Their names start with an underscore. So you can't create them yourself but they come in very handy. Escape' the doublequotes inside the string-doublequotes with a slash otherwise PHP won't understand it.
Fields have to be seperated by a comma.
Every tablerow is a form now. The server will generate warnings if you exceed fieldsizes, use wrong data for the fieldtypes or, for the index, use a non-unique value. You'll also get a message from the script on the status of your Update-operation. Notice that this is an inefficient query as it updates all the fields in the row instead of only the ones you've changed. It is also good practice to ask for confirmation from the user before really updating the table.