Processing fields in PHP

I noticed that some students are trying to refer to the form fields by their names in the php form processing program. You cannot do that and have to refer to the variables by also mentioning that the system has to get them from the POST flow that you use to post your html m to the server for processing by PHP.
Therefore all names from the html form will look like $_POST["your field name"] and should be addressed in such way (with added prefix $_POST[...]).