小编Dle*_*ler的帖子

这是我的用户注册数据库的方式，但我的问题是：如何防止数据库拥有相同用户名的副本，或者换句话说，如果他们的用户名，我如何向用户提示“您的用户名已经存在”存在于数据库中。

 <?php

        $error = ""; // error
        $GoodJob = ""; 
        //When submit button is pressed, send data.  

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

            if (empty($_POST['username']) || empty($_POST['email']) || empty($_POST['password'])) {

                $error = "<br>Please insert only letters or numbers.";

            }else{

            // Define username, firstname, lastname, email and password.
            $username = $_POST['username'];
            $firstname = $_POST['firstname'];
            $lastname = $_POST['lastname'];
            $email = $_POST['email'];
            $password = $_POST['password'];

            // Define database information
            $hostnameV = "***";
            $usernameV = "***";
            $passwordV = "***"; 
            $databaseV = "***"; 



            //connection to the database
            $connection = mysql_connect($hostnameV, …