Adding a field to a file based on checkbox value

I am doing a PHP assignment where I am getting input that uses some checkboxes and stores the information in a file.

Let's say I have a checkbox

 Active: <input type="checkbox" name="preference" value="Pepperoni" checked="checked>

When I say something like:

$Preference1 = $_POST['preference'];

What is stored in $Preference1? Does it store information that tells whether the box was checked or not, or does it contain "Pepperoni"? Is there a way to check to see if a box is checked? Like,

Pesudocode

if Preference1 is checked
   add Pepperoni to the file;

if Preference2 is checked
   add Sausage to the file;

I did work with radio buttons but those are a bit different since only one of those can be checked at a time, but for checkboxes, multiples boxes and be selected. Is there a way to see if a button is checked and if so, how would I do that? Thanks.