想要从基于下拉列表选择数据库中获取数据使用php

Question

我有一个php文件和mysql数据库字段名为planname和价格，我想从数据库的所有planname下拉列表和根据计划名称的价格特定计划名称应显示在下面的文本框中。 这是我的PHP文件;

<?php 

$servername = xxxxxxx; 
$username = xxxxxx; 
$password = xxxxxx"; 


try { 
    $conn = new PDO("mysql:host=$servername;dbname=vnet", $username, $password); 
    // set the PDO error mode to exception 
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION); 
    echo "Connected successfully"; 
    } 
catch(PDOException $e) 
    { 
    echo "Connection failed: " . $e->getMessage(); 
    } 




$sql="SELECT id,planname,price FROM plan"; 

/* You can add order by clause to the sql statement if the names are to be displayed in alphabetical order */ 

echo "<select name=planname value=''>Plan Name</option>"; // list box select command 

foreach ($conn->query($sql) as $row){//Array or records stored in $row 

echo "<option value=$row[id]>$row[planname]</option>"; 


/* Option values are added by looping through the array */ 

} 

echo "</select>";// Closing of list box 

if(isset($_REQUEST['planname'])){ 
    // connection should be on this page 
    $sql = mysql_query("select price from plan where planname =".$_REQUEST['planname']); 
    $res = mysql_fetch_assoc($sql); 
    echo $res['price'];die; 
} 

echo '<input type="text3" name="price[]" id="price" value="', $row['price'], '" disabled="disabled" />'; 

?> 

我在下拉列表中，但不能根据planname dynamically.can任何人帮我出这个拿到的价格？