MySQL/PHP - Passing/Using Multiple Variable Values

I would be grateful for any help with this problem using PHP with a MySQL database....

I have a search form facility based on a drop-down list (of products). I want searchers to be able to select more than one product from the drop-down to be used as search criteria. I have set up the form to allow multiple selections.

The variable values get passed in the form, e.g.

AdvancedResults.php?Products=AC&Products=BA

but it is only the second value that is 'held' and can be used.

How can I pass multiple values for a single variable to enable results from the database to be generated on a 'this value AND that value' basis?????

Many thanks

you can't select multiple items in a drop-down list, only a regular list box.

http://www.phpnoise.com/tutorials/6/4

you have to make the form item an array