How to auto retrieve data from mysql database in select box using another select box?

I have a following MySQL Table named customers:-



In the above table -> company_name: there are duplicate records, that means same company having different contact person.

Following is my question:

I want two select box in codeigniter application:
1st Select box should retrieve unique values from company_name field of above table.
2nd Select box should retrieve the name of contact_person where company_name is selected in the 1st select box.

(For example: If ABC Company is selected in 1st Select box, then 2nd Select box should show only John Doe and Jane white)

How can I do this in codeigniter? It would be a great help if anyone can provide me codeigniter code for above.

Thanks in advance.