Mate

<?php
    include 'connect.php';

    //User data;
    $username = $_POST["username"];
    $password = md5($_POST["password"]);
    $email = $_POST["email"];

    //Check user
    $sql = "SELECT * FROM user WHERE username = '$username' OR email = '$email'";
    $result = $conn->query($sql);
    $row = $result->fetch_assoc();
    if(empty($row)){
        //Mysql query
        $sql = "INSERT INTO user (username, password, email)
        VALUES ('$username', '$password', '$email')";

        if ($conn->query($sql) === TRUE) {
            echo "Register successfull";
        } else {
            echo "Error: " . $sql . "<br>" . $conn->error;
        }
    }else echo "A user is already registered with this username or email address";
?>