Using php + javascript to create a level-2 cascade menu

I used asp + js for the level-2 cascade menu last year, but now I suddenly found that I forgot it, in addition, I couldn't find any programs written using asp. I found that [emot] sweat [emot] was dizzy, so I searched the internet and searched for it for a long time. I found that the programming languages on the Internet were different, and they are all very complicated. is it necessary to make such a level-2 cascade menu so complicated? So I want to write a simple one again. After about half an hour of thinking, I completed the design and production of the level-2 cascade menu. The general idea is as follows: in order not to refresh the previous page, I used iframe to dive into a second-level subpage to read data in the database and finally pass the desired data to the parent page, select and transfer data.

The main program code is as follows (some code has been modified but does not affect the function ):
Parent page reg.html:



Sub-page city. php: