php-linkage drop-down list with PHP, HTM transfer value

php-linkage drop-down list and PHP, HTM transfer value
Dis.htm

Destination CityContinents
  
    $con [Name]
  
    Countries
  
    $cou [Name]
  
    Regional
  
    $dis [Name]
  
    Provinces
  
    $pro [Name]
  
    City
  
    $CI [Name]
  
    

dis.php

......
$country =getbycontent ($content);
$pro =getbycountry ($country);
......

I want the first drop-down list to appear after the second drop-down list is selected, and the list item for the second drop-down list is searched based on the option selected in the first drop-down list. How can I send a value between these two files?
------to solve the idea----------------------
demo.php

 
  
Demo

server.php

 $parent = Isset ($_get[' parent ')? $_get[' parent ']: 0;
$type = isset ($_get[' type ')? $_get[' type ']: 0;

$sel = Array ();
$sel [1][1] = Array (', ' Array (' ID ' =>1, ' name ' = ' China '), array (' ID ' =>2, ' name ' = ' Japan '));
$sel [=>3] = Array (', ', array (' ID '), ' name ' = ' UK '), array (' ID ' =>4, ' name ' = ' France '));

$sel [2] = array ();
$sel [2][1] = Array (', ' Array (' ID ' =>1, ' name ' = ' South '), array (' ID ' =>2, ' name ' = ' North '));
$sel [2][2] = Array (', ' Array (' ID ' =>3, ' name ' = ' South '), array (' ID ' =>4, ' name ' = ' North '));
$sel [2][3] = Array (', ' Array (' ID ' =>5, ' name ' = ' South '), array (' ID ' =>6, ' name ' = ' North '));
$sel [2][4] = Array (', ' Array (' ID ' =>7, ' name ' = ' South Bank '), array (' ID ' =>8, ' name ' = ' North Shore '));

$sel [3] = array ();
$sel [3][1] = Array (', ' Array (' ID ' =>1, ' name ' = ' China 1 '), array (' ID ' =>2, ' name ' = ' Chinese Province 2 '));
$sel [3][2] = Array (', ' Array (' ID ' =>3, ' name ' = ' China 3 '), array (' ID ' =>4, ' name ' = ' Chinese Province 4 '));
$sel [3][3] = Array (', ' Array (' ID ' =>5, ' name ' = = ' Japan 1 '), array (' ID ' =>6, ' name ' = ' Japan 2 '));
$sel [3][4] = Array (', ' Array (' ID ' =>7, ' name ' = = ' Japan 3 '), array (' ID ' =>8, ' name ' = ' Japan 4 '));
$sel [3][5] = Array (', ' Array (' ID ' =>9, ' name ' = ' UK Province 1 '), array (' ID ' =>10, ' name ' = ' uk Province 2 '));
$sel [3][6] = Array (', ' Array (' ID ' =>11, ' name ' = ' UK Province 3 '), array (' ID ' =>12, ' name ' = ' uk Province 4 '));
$sel [3][7] = Array (', ' Array (' ID ' =>13, ' name ' = = ' French province 1 '), array (' ID ' =>14, ' name ' = ' French Province 2 '));
$sel [3][8] = Array (', ' Array (' ID ' =>15, ' name ' = = ' French province 3 '), array (' ID ' =>16, ' name ' = ' French Province 4 '));

$sel [4] = array ();
$sel [4][1] = Array (', ' Array (' ID ' =>1, ' name ' = ' China City One '), array (' ID ' =>2, ' name ' = ' China 12 '));
$sel [4][2] = Array (', ' Array (' ID ' =>3, ' name ' = ' "China"), Array (' ID ' =>4, ' name ' = ' 22 ' in China city));
$sel [4][3] = Array (', ' Array (' ID ' =>5, ' name ' = ' China '), array (' ID ' =>6, ' name ' = ' China 32 '));
$sel [4][4] = Array (', ' Array (' ID ' =>7, ' name ' = ' Chinese city '), array (' ID ' =>8, ' name ' = ' 42 ' in China city));
$sel [4][5] = Array (', ' Array (' ID ' =>9, ' name ' = = ' japan City One '), array (' ID ' =>10, ' name ' = ' Japan 12 '));
$sel [4][6] = Array (', ' Array (' ID ' =>11, ' name ' = ' Japanese '), array (' ID ' =>12, ' name ' = ' Japan 22 '));
$sel [4][7] = Array (', ' Array (' ID ' =>13, ' name ' = = ' Japan '), array (' ID ' =>14, ' name ' = ' Japan 32 '));
$sel [4][8] = Array (', ' Array (' ID ' =>15, ' name ' = ' Japan '), array (' ID ' =>16, ' name ' = ' Japan 42 '));
$sel [4][9] = Array (', ' Array (' ID ' =>17, ' name ' = = ' uk City One '), array (' ID ' =>18, ' name ' = ' UK City 12 '));
$sel [4][10] = Array (', ' Array (' ID ' =>19, ' name ' = ' "English city"), Array (' ID ' =>20, ' name ' = ' UK City 22 '));
$sel [4][11] = Array (', ' Array (' ID ' =>21, ' name ' = ' "UK City"), Array (' ID ' =>22, ' name ' = ' UK City 32 '));
$sel [4][12] = Array (', ' Array (' ID ' =>23, ' name ' = ' "UK City"), Array (' ID ' =>24, ' name ' = ' UK City 42 '));
$sel [4][13] = Array (', ' Array (' ID ' =>25, ' name ' = = ' French city One '), array (' ID ' =>26, ' name ' = ' France City 12 '));
$sel [4][14] = Array (', ' Array (' ID ' =>27, ' name ' = ' "French city"), Array (' ID ' =>28, ' name ' = ' France City 22 '));
$sel [4][15] = Array (', ' Array (' ID ' =>29, ' name ' = ' + ' French city '), array (' ID ' =>30, ' name ' = ' France City 32 '));
$sel [4][16] = Array (', ' Array (' ID ' =>31, ' name ' = ' "French city"), Array (' ID ' =>32, ' name ' = ' France City 42 '));

$list = $sel [$type] [$parent];

$ret = Array ();
$ret [' success '] = true;
$ret [' list '] = $list;
echo Json_encode ($ret);

?>

Server.php can be converted into a database, so it is more convenient.

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