Problems related to PHP + mysql implementation of level-2 cascading drop-down boxes

For more information about how to implement level-2 cascade drop-down boxes in PHP + mysql, visit prawn. PHP + mysql implements level-2 cascade drop-down boxes to solve the problem of many classes, select the year of admission before selecting the class that belongs to this year. & Nbsp; the year in the first drop-down box can be read, but after a year is selected, nothing is displayed in the class drop-down box. the code is as follows, please take a look at the problem of implementing level-2 cascade drop-down boxes in PHP + mysql.
Ask the prawn door, PHP + mysql implementation level 2 cascade drop-down box, solve the class many cases, first select the admission year and then select the class of this year.
The year in the first drop-down box can be read, but after a year is selected, nothing is displayed in the class drop-down box. the code is as follows. Please check out the problem. thank you.
/*************************************** ********
**
** Database: database name (alumni), Data Table (enteryear, banji)
** Fields in table enteryear: enter_id (id number) and entertime (year)
** Fields in the banji table: id (id number), classnum (class number), and enteryear (year)
**************************************** *******/
// ******************* Select a database connection ***************
$ Link = mysql_connect ("localhost", "root ","")
Or die ("cocould not connect:". mysql_error ());
Mysql_select_db ("alumni") or die ("cocould not select database ");


// ******************* Extract province information ***************** *
$ SqlSel = "select * from enteryear order by enter_id ";
$ Result = mysql_query ($ sqlSel) or die ("Query failed:". mysql_error ());

$ Forum_data = array ();
While ($ row = mysql_fetch_array ($ result ))
{
$ Forum_data [] = $ row;
}
// Print_r ($ forum_data );
Mysql_free_result ($ result );

// ************** Obtain city information **************
$ SqlSel2 = "select * from banji order by id desc ";

If (! ($ Result2 = mysql_query ($ sqlSel2 )))
{
Die ('could not query t_city list ');
}

$ Forum_data2 = array ();
While ($ row2 = mysql_fetch_array ($ result2 ))
{
$ Forum_data2 [] = $ row2;
}

Mysql_free_result ($ result2 );
?>