Need some help folks....

I have an array of categories that I need to turn into text links.... the array looks like this but contains many more listings.... "online" =>"Online Business "rent_auto" =>"Automobile / Boat Rentals "retail_antique" =>"Antique "food_bakery" =>"Bakeries "serv_apartment" =>"Apartment Locators "man_apparel" =>"Apparel "fin_accounting" =>"Accounting "other_agg" =>"Agriculture you get the idea.... What I need to do is check the KEY of the array, and when the key changes, I need to start a new section in the links like so... Online Businesses..... ......... ........ Rentals ......... ......... Retail Etc.... How do you suppose I should do this? The trick to the KEY is to check the first part of the KEY and if it changes, then to change the section.... but how? Much thanks... Dallas K.