您可以生成选择窗体：

<?php 
$req = $pdo->query('SELECT * FROM table');
$rep = $req->fetchAll(); ?>
<select>
foreach($rep as $row) { ?>
  <option value="<?= $row['country'] ?>"><?= $row['country'] ?></option>
<? } ?>
</select>
<?php foreach($rep as $row) { 
  <input style="display:none" id="<?= $row['country'] ?>" value="<?= $row['capital'] ?>" />
<?php } ?>

因此，您将拥有selectwithallcountry，以及每个首都的一个输入，其国家作为id，这样您就可以用javascript显示它了：（jqueryexample）

<script>
$('select').change(function() {
  $('input:visible').toggle(); 
  $('input[id='+$(this).val()+']').toggle();
});
</script>

您可以使用中的以下代码从数据库中获取国家和其他国家的首都 php ```
//This is where you put the fetched data
$entries = Array();

//Make new connection
$connection = new mysqli("127.0.0.1", "username", "password", "databasename");

//Create prepared statement
$statement = $connection->prepare("SELECT country, capital FROM table");
$statement->bind_result($country, $capital);

//Put the fetched data in the result array
while($statement->fetch()) {
$entry = Array();

$entry['country'] = $country;
$entry['capital'] = $capital;

$entries[] = $entry;
}

//Close the statement and connection
$statement->close();
$connection->close();

接下来，创建html `select` 物体。重要的是国家的顺序和首都的顺序保持一致。