我的 AJAX 代码：

$('#name').keyup(function() {
        var usercheck = $(this).val();
                $('#nameAvailability').html('<img src="../SPR/assets/img/loading.gif" width="300" />'); //this part is working
                $.post("../SPR/backend/username_availability_check.php", {user_name: usercheck} ,  
                function(data)
                {
                if (data.status == true)
                {
                $('#nameAvailability').parent('div').removeClass('has-error').addClass('has-success');

                } else {
                $('#nameAvailability').parent('div').removeClass('has-success').addClass('has-error');
                }
                $('#nameAvailability').html(data.msg); // not working
                } ,'json');

        });

我的php代码：

<?php
require("connection.php");
if(isset($_POST['user_name']) && $_POST['user_name'] != '')
    {
        $response = array();
        $username = mysqli_real_escape_string($conn,$_POST['user_name']);
        echo $username;
        $sql  = "select username from users where users.username='".$username."'";
        $res    = mysqli_query($conn, $sql);
        $count  = mysqli_num_rows($res);
        if($count > 0)
        {
            $response['status'] = false;
            $response['msg'] = 'Username already exists.';
        }
        else if(strlen($username) < 6 || strlen($username) > 15){
            $response['status'] = false;
            $response['msg'] = 'Username must be 6 to 15 characters';
        }
        else if (!preg_match("/^[a-zA-Z1-9]+$/", $username))
        {
            $response['status'] = false;
            $response['msg'] = 'Use alphanumeric characters only.';
        }
        else
        {
            $response['status'] = true;
            $response['msg'] = 'Username is available.';
        }
         echo json_encode($response);
         echo $response;
    }
?>

我在index.php中使用了session_start（），用户在输入字段中输入了他的用户名，id为“name”
我已经检查了给定的php代码，通过从数据库传递一个自定义的用户名来单独运行它，它工作正常。所以可能是 AJAX 代码有问题。