Extracting from arrays - alphabetically

Hi, A simple problem for you guys (I think), I have 2 arrays - firstname and lastname, both have the same associative index - lastname, I have then performed asort on both to order them alphabetically. Now I just need to extract and display the array elements that have the associative index beginning with A or B or whatever is defined by $query - thus displaying only names where the lastname begins with A etc. How do I do this? Thanks for any help you can offer. Lar