MySQLi插入不正常

if(isset($_POST['submit']))
{
    extract($_POST);

    $query = "INSERT INTO Messages VALUES(null,'$user_id','$subject','$msg')";
    mysqli_query($conn,$query) or die (mysqli_error($conn));
}

You can try this code (check if $_POST is not empty first and then execute):

if ( !empty($_POST['submit'])) {
    $query = "INSERT INTO"
        ."Messages"
        ."VALUES"
           ."(null,'$user_id','$subject','$msg')";
    mysqli_query($conn,$query) or die (mysqli_error($conn));
}