JavaScript代码未被执行

我正在研究这个项目，我正在尝试获取一个返回值，以便我可以根据客户端选择的内容自动填充我的输入框。

但是，此代码未执行，我不知道为什么。当我删除src="jquery area" $(#dropdown).on时，这是一个未定义的方法;不知道该怎么办。

<script  type="text/javascript"  src="http://code.jquery.com/jquery-latest.min.js">
//$.post(url, [data], [callback], [callback type])
    ("#dropdown").on('change', function() {//when you select something from the dropdown function run and will switch the data
        $.post("backgroundScript.php", {
                uid: $(this).val()
            },
             function(data) {
                 $("#first").val(data.first);
               $("#last").val(data.last);
               // etc.;
            }, 'json'
        );
    });
</script>

这是我的完整代码

try {  
  # MySQL with PDO_MYSQL  
  $DBH = new PDO("mysql:host=$hostname;dbname=$database", $username, $password);  
  $DBH->setAttribute( PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION );
  //$DBH->prepare('SELECT first FROM contacts');
}  
catch(PDOException $e) { 
    echo "I'm sorry, I'm afraid I can't do that.";  
    file_put_contents('PDOErrors.txt', $e->getMessage(), FILE_APPEND);   
}  
//get query
$FNresult=$DBH->query('SELECT first FROM contacts'); 
//set fetch mode
$FNresult->setFetchMode(PDO::FETCH_ASSOC);
$dropdown = "<select name='contacts' id='contacts' >";
while($row =$FNresult->fetch()) {
  $dropdown .= "'r'n<option value='{$row['first']}'>{$row['first']}</option>";
 // echo getLN();
}
$dropdown .= "'r'n</select>";
echo $dropdown;
//}
/*
//                  Get last name
function getLN(){
    $query = "SELECT last FROM contacts";
    $LNresult=mysql_query($query);
    $last;
    while($row = mysql_fetch_assoc($LNresult)) {
        $last = "{$row['last']}";
    }
    echo $last;
}//end getLN
*/
$DBH = null; 
?>
<!-- javascript on client-side -->
<script  type="text/javascript"  src="http://code.jquery.com/jquery-latest.min.js">
//$.post(url, [data], [callback], [callback type])
    ("#dropdown").on('change', function() {//when you select something from the dropdown function run and will switch the data
        $.post("backgroundScript.php", {
                uid: $(this).val()
            },
             function(data) {
                 $("#first").val(data.first);
               $("#last").val(data.last);
               // etc.;
            }, 'json'
        );
    });
</script>

<script type="text/javascript" src="http://code.jquery.com/jquery-latest.min.js">
/*("#dropdown").on('connection', function (stream) {
  console.log('Ah, we have our first user!');
});*/</script>
<form action="insert.php" method="post">
First Name: <input type="text" id="first" name="first"><br>
Last Name: <input type="text" id="last"><br>
Phone: <input type="text" id="phone"><br>
Mobile: <input type="text" id="mobile"><br>
Fax: <input type="text" id="fax"><br>
E-mail: <input type="text" id="email"><br>
Web: <input type="text" id="web"><br>
<input type="Submit">
</form>

这是我在输出页面上新编辑的脚本 =

<script type="text/javascript"
     src="http://code.jquery.com/jquery-latest.min.js"></script>
<script>    
//$("#dropdown-parent").on('change','#dropdown', function() { // replace dropdown-parent
    $("#contacts").on('change','#dropdown', function() {
        $.post("backgroundScript.php", { 
                uid: $(this).val() 
            }, 
             function(data) { 
                 $("#first").val(data.first); 
                 $("#last").val(data.last); 
                 // etc.; 
            }, 'json' 
        ); 
    }); 
</script>

这是 backgroundScript 的 PHP 文件.php =

<?
// background script
// retrieve data based on $_POST variable, set to $returnArray
$returnArray = $_POST[array(
         'first' => firstName,
         'last' => lastName,
)];
/****************************
 * the structure of returnArray should look something like
     array(
         'first' => firstName,
         'last' => lastName,
     )*/
echo json_encode($returnArray);
?>

此文件将发送信息，因此JavaScript将用指定区域中保存的内容替换表单字段