Grouping ZIP codes into ranges
I think you could use a one-liner like:

$zip_arr_id = floor(($zipInt/1000)/5 )*5;


which could correspond to the $zips${$zip_arr_id}_array (unsure if this is exactly right) that you're saving the org ID's to. Look into variable variables in php.

Riffing on this idea could save you a lot of lines in your code.