How do I populate 2 drop down menu's from my database, based upon a selection from the first menu

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Java

PHP

Database structure:
-MassWarVetsADM (database name)
-Business (business table)
-BName | Primary Key
-Category

-Category (category table)
-Category | Foreign key referencing parent Business:Category


Using php & javascript only, can someone assist me in populating 2 select boxes with the following stipulations?

First a user will select a business category (Category:Category) from a select box on my homepage, that will have the categories loaded from the (Category:Category) table/column upon loading the homepage.

Then a second select box will populate after a category selection is made, all of the Business names(Bname) that share that category. Your help is greatly appreciated!