如何使用jQuery/AJAX和PHP/MySQL选择第一个下拉菜单来填充第二个下拉菜单?

I am trying to create a dynamic set of dropdown boxes, using jQuery/AJAX and PHP/MySQL. The first dropdown box will be populated when the page loads based on values from a database. The second dropdown box should display a set of values based on the selection from the first dropdown box. I know there have been similar questions asked on here before, but I haven't found a solution that matches my scenario.I am trying to create a dynamic set of dropdown