检查用户名是否存在，如果是帐户，则发送验证邮件

Question

如果我使用mysqli_real_escape_string函数来转义SQL注入，那么我的查询会出错。如果创建了用户，我想发送带有网址和密钥的激活电子邮件，以便我使用$_GET阅读并向数据库发送帐户已激活（0＆gt; 1）

connection.php包含在header.php文件中。

<?php
    include("inc/header.php");
?>

<?php
    if(isset($_POST["submit"])) {
        $username = trim($_POST["username"]);
        $password = trim($_POST["password"]);

        if(empty($username) && empty($password)) {
            echo "Please fill in all the details";
        }

        $username_check_sql = "SELECT * FROM user WHERE username = '".mysqli_real_escape_string($username)."'";
        $username_check_query = mysqli_query($connection, $username_check_sql);
        $username_row_count = mysqli_num_rows($username_check_query);

        if($username_row_count != 0) {
            echo "Username already exist";
        }

        $password_secure = md5($password);
        $new_user_sql = "INSERT INTO user(username, password, activated) VALUES('".mysqli_real_escape_string($username)."', 
            '".mysqli_escape_string($password_secure)."', 0)";
        $new_user_query = mysqli_query($connection, $new_user_sql);

        $key = uniqid();

        $to = $username;
        $subject = "activation required";
        $header = "from: test";
        $message = "Your Confirmation link\n";
        $message = "Click on this link to activate your account\n";
        $message = "localhost/examen/activate_account.php?key=".$key."";

        $sendmail = mail($to, $subject, $message, $header);

        header("location: registreren_success.php?account=made");
        exit;
    }

?>

    <div class="registratie-container">
        <div class="container">
            <form method="POST" action="<?php echo htmlentities($_SERVER["PHP_SELF"]);?>">
                <div class="form-group">
                    <label for="username">Username</label>
                    <input type="email" name="username" id="username" placeholder="Username">
                </div>
                <div class="form-group">
                    <label for="password">Password</label>
                    <input type="password" name="password" id="password" placeholder="Password">
                </div>
                <div class="form-group">
                    <input type="submit" name="submit" id="submit" value="Registreer">
                </div>
            </form>
        </div>
    </div>

<?php
    include("inc/footer.php");
?>