我无法检查数据库中是否已经存在用户名。我浏览了这里回答的现有问题。他们都没有解决我的问题。当我执行时，它显示“cannotselectusername fromtable”，这是我在die块中给出的。代码如下。

<?php
    $username = $_POST['user_name'];
    $password = $_POST['pass_word'];
    $host = "localhost";
    $db_username = "root";
    $db_password = "";
    $db_name = "my_db";
    //create connection
    $conn = @new mysqli($host, $db_username, $db_password, $db_name);
    if (isset($_POST["submit"]))
    {
        # code...
        //check  connection established or not
        if ($conn->connect_error)
        {
            die("Not Connected to DB"); 
        }
        else
        {
            $query = "SELECT 'usernamedb' FROM 'registration' WHERE usernamedb='$username'";
            $result = mysqli_query($conn, $query) or die('Cannot select username from table');
            if (mysqli_num_rows($result)>0)
            {
                $msg.="This username already exist. try Another !!";
            }
            else
            {
                $insert = "INSERT INTO 'registration'('id', 'usernamedb', 'password') VALUES ([$username],[$password])";
                $insert_result = mysqli_query($conn,$insert) or die('INSERTION ERROR');
            }
        }
        $conn->close();
    }
?>

希望有人能回答我。