How to use php + javascript to create a level-2 cascade menu _ PHP Tutorial

Use php + javascript to create a 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 from the database, finally, the general idea of passing the desired data to the parent is as follows: in order not to refresh the previous page, I used iframe to dive into a second-level subpage to read the data in the database, finally, pass the desired data to the parent page to select and transfer the 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: