I'll give you an example and hopefully it will help. You can use either a html selection or checkbox or any other form in put for that matter.
Let's say you present the user with a list of checkbox to pick what person in their address box that they want to 'tell a friend' about a page or forward an email to. Presented to the user is a list of checkbox
...
<input type='checkbox' value='user1@domain.com' name='ToList[]'> user1@domain.com
<input type='checkbox' value='user2@domain.com' name='ToList[]'> user1@domain.com
<input type='checkbox' value='user3@domain.com' name='ToList[]'> user1@domain.com
...
Note the empty array notation for the name attribute. This tells PHP on submit that to expect an array and fill it with value for every selected item on submit. So, when the form is submitted, the server side code will have the variable name ToList is scope or you can access it by $HTTP_POST_VARS['ToList']. You can also do this any form input and just assign the name = array[]. (Note: I haven't tried with all form input types, just selection, radio, hidden and checkbox)
hope this helps