Using 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 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:

<Iframe src = "city. php "width =" 300 "height =" 22 "frameborder =" 0 "scrolling =" no "> </iframe> <input name =" city "type =" hidden "id = "city" value = ""/>

Sub-page city. php:

<Script language = "javascript" type = "text/javascript">
Function goto (n ){
This. location. href = "city. php? Sh_id = "+ n;
}
</Script>

<Select name = "sh" onchange = "goto (this. value)">
<Option> select the province and city </option>
<? Php
Include_once ("db. php ");
$ SQL = "select * from province order by sh_id asc ";
$ Result = mysql_query ($ SQL );
While ($ row = mysql_fetch_assoc ($ result )){
?>
<Option value = "<? Echo $ row ["sh_id"];?>" <? If ($ _ GET ["sh_id"] = $ row ["sh_id"]) {echo 'selected = "selected" ';}?>> <? Echo $ row ["sh_name"];?> </Option>
<? Php
}
?>
</Select>
<Select name = "city" onchange00000000parent.doc ument. getElementById ('city'). value = this. value ">
<Option> select your city </option>
<? 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 )){
?>
<Option value = "<? Echo $ row ["city_name"];?> "> <? Echo $ row ["city_name"];?> </Option>
<? Php
}
}
?>
</Select>