Session variables and arrays

I'm having a problem with session variables and arrays. I'm building a multi step sign up form and I need to store all the variable until the final step, when they are inputed to a database. I thought the best way to do this would be to store the contents for $HTTP_POST_VARS in session variables. This works fine for everything except for my array of checkboxes. When I do a print_r($_SESSION) it displays the following for the sessionvariable assigned to the checkboxes: [categories] => Array ( [0] => '1'; [1] => '12'; ... Etc ... But when I run any sort of array function (is_array, foreach) on $_SESSION['categories'] it appears that it is a scalar, not an array. Any ideas? - Ryan