php回傳json資料的demo如下，請問是什麼原因導致該問題？

<code>&lt;?php
// Include ezSQL core
include_once('../include/ez_sql_core.php');

// Include ezSQL database specific component
include_once('../include/ez_sql_postgresql.php');
$username='';
$password='';
$result='';
if(!empty($_POST)){
    $username = $_POST["username"];
    $password = $_POST["password"];
}else{
    
}

$db = new ezSQL_postgresql('i2soft', 'i2s@Shanghai', 'sdata', '127.0.0.1');


///$user = $db-&gt;query("select id from user2 where name = 'admin' and password ='admin'");
//$user_details = $db-&gt;get_row(null, OBJECT, 0);
//var_dump($user_details) ;

$sql = sprintf("select * from user2 where name='%s' and password='%s'",$username,$password);
//echo $sql;
//select * from user2 where name='admin' and password='admin'
$user=$db-&gt;get_results($sql);

//var_dump($user);
//array(1) { [0]=&gt; object(stdClass)#5 (3) { ["id"]=&gt; string(1) "1" ["name"]=&gt; string(5) "admin" ["password"]=&gt; string(5) "admin" } }
if($user){
  $result='0';
   
}else{
   $result='-1';
  
}

$arr = array ('a'=&gt;1,'b'=&gt;2,'c'=&gt;3,'d'=&gt;4,'e'=&gt;5);
echo json_encode($arr);
?&gt;

</code>

php回傳json數據，若含有

<code>$user=$db-&gt;get_results($sql);  </code>

這行程式碼ajax就回傳error。

若去掉這行程式碼，則ajax結果是success

html程式碼如下：

<code>&lt;!DOCTYPE html&gt;
&lt;html lang="zh-CN"&gt;

    &lt;head&gt;
        &lt;meta charset="utf-8"&gt;
        &lt;meta http-equiv="X-UA-Compatible" content="IE=edge"&gt;
        &lt;meta name="viewport" content="width=device-width, initial-scale=1"&gt;
        &lt;meta name="description" content=""&gt;
        &lt;meta name="author" content=""&gt;
        &lt;link rel="icon" href="../media/image/favicon.ico"&gt;
        &lt;title&gt;英方I2Active&lt;/title&gt;
        &lt;!-- Bootstrap core CSS --&gt;
        &lt;link href="../media/bootstrap/css/bootstrap.min.css" rel="stylesheet"&gt;
        &lt;!-- Custom styles for this template --&gt;
        &lt;link href="../media/css/login.css" rel="stylesheet"&gt;
        &lt;script type="text/javascript" src="../media/js/jquery-1.9.1.js"&gt;&lt;/script&gt;
    &lt;/head&gt;

    &lt;body&gt;
        &lt;div class="container"&gt;
            &lt;form class="form-signin" role="form" action=""&gt;
                &lt;img src="../media/image/logo.png" /&gt;
                &lt;h2 class="form-signin-heading"&gt;I2Active登录&lt;/h2&gt;
                &lt;input  id="username" type="text" class="form-control" placeholder="User name" required autofocus &gt;
                &lt;input id="password" type="password" class="form-control" placeholder="Password" required&gt;
                &lt;div class="checkbox"&gt;
                    &lt;label&gt;
                        &lt;input type="checkbox" value="remember-me"&gt; 记住密码
                    &lt;/label&gt;
                &lt;/div&gt;
                &lt;button id="login" class="btn btn-lg btn-primary btn-block" &gt;登录&lt;/button&gt;
            &lt;/form&gt;
        &lt;/div&gt;
        &lt;!-- /container --&gt;
        &lt;script type="text/javascript"&gt;  
            
            $("#login").click(function () {
                var username = $("#username").val();
                var password = $("#password").val();
                $.ajax({
                    url: '../controller/login2.php', // 跳转到 action  
                    data: {
                        'username': username,
                        'password': password
                    },
                    type: 'post',
                    cache: false,
                    dataType: 'json',
                    success: function (data) {
                        alert('ajax success');
                    },
                    error: function () {
                      alert('ajax error');
                    }
                })
            });
        
        &lt;/script&gt;
    &lt;/body&gt;

&lt;/html&gt;</code>