PHP+Javascript Use php+javascript to realize the production of secondary cascading menus

The general idea is this: In order to prevent the previous page from refreshing, I used an iframe to sneak into a secondary sub-page to read the data in the database, and finally passed the desired data to the parent page to complete the data processing. Select and transfer.
The main program code is as follows (part of the code has been changed, but it does not affect the function):
Parent page reg.html:

Sub-page city.php:

Please select your province and city
include_once(”db.php”);
$sql=”select * from province order by sh_id asc”;
$result=mysql_query($sql);
while($row=mysql_fetch_assoc($result)){
?>
” < ;? if($_GET["sh_id"]==$row["sh_id"]){echo 'selected="selected"';}?>>
}
?>

Select your city
< ;?php
if(!empty($_GET[”sh_id”])){
//echo “ok”;
$sql=”select * from city where sh_id=”.$_GET[”sh_id”].” order by city_id asc”;
$result=mysql_query($sql);
while($row=mysql_fetch_assoc($result)){
?>
”>
}
}
?>

The above has introduced PHP+Javascript to use php+javascript to realize the production of secondary cascading menus, including the content of PHP+Javascript. I hope it will be helpful to friends who are interested in PHP tutorials.