在ios应用程序中，我试图获取用户输入并将其提交到mysql数据库，但是数据库没有更新。以下是用于提交数据的swift函数：

func submit_data(){
    let request = NSMutableURLRequest(url: NSURL(string: "omitted")! as URL)
    request.httpMethod = "POST"
    let postString = "a=\(bar_name)&b=\(user_data)"
    request.httpBody = postString.data(using: String.Encoding.utf8)     
    let task = URLSession.shared.dataTask(with: request as URLRequest) {
        data, response, error in      
        if error != nil {
            print("error=\(error)")
            return
        }           
        print("response = \(response)")        
        let responseString = NSString(data: data!, encoding: String.Encoding.utf8.rawValue)
        print("responseString = \(responseString)")     
        print(postString)    
    }
    task.resume()  
}

以下是我的php代码：

<?PHP
//Create connection
$con=mysqli_connect("localhost","username","password","covermeo_coverme_data");
//check connection

if(mysqli_connect_errno())
{
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}

$sql = "INSERT INTO User_Data (Bar, Cover)
VALUES ('".$_POST["a"]."','".$_POST["b"]."')";
$result = mysqli_query($conn,$sql);

// Close connections
mysqli_close($con);
?>

下面是mysql数据库的截图。