checkboxes & php

If I have 3 checkboxes labeled "user", but each one has a seperate value, how do I get the values of each checkbox? For example, how would I tell if the following checkboxes are checked? <input type="checkbox" name="user" value="1"> <input type="checkbox" name="user" value="2"> <input type="checkbox" name="user" value="3"> <input type="submit" name="Submit" value="Submit">