looping through an array

hi... i have a combo box that is filled with a query from an sql table... this is its logic: if $_SESSION['guestbook'] doesnt exist then query database, fill $_SESSION['guestbook'] with its values from the db, turn the "value" part of the <option> into the values from $_SESSION['guestbook']['Number'] and when done close the box... if $_SESSION['guestbook'] already exists and is filled then use those existing values to do the above (fill the combobox) the part i have the problem with is how to get $_SESSION['guestbook'] to do the above if it already exists... i tried using while($_SESSION['guestbook']) but it proves in php's mind to be blank or that whole statement gets ignored... help...