用户已被带走？

I have this code:

$username = $_POST["username"];
$password_input = $_POST["password"];
$password = md5($password_input);
$email_input = $_POST["emailaddress"];
$email = md5($email_input);

if (!($stmt  = $con->prepare("INSERT INTO `users` (`username`,`password`,`email_address`) VALUES (?,?,?)")) || !is_object($stmt)) {
    die( "Error preparing: (" .$con->errno . ") " . $con->error);
}
$stmt->bind_param('sss', $username, $password, $email);
$stmt->execute();
$stmt->close();
echo "User has been Created! Feel free to login - <a href='login.php'><span class='button color_blue'>Login</span></a>";

Within the SQL database an email/username can only be used once (UNIQUE) and I wondered if there was a way to change the echo to only appear if the data was successfully added and then a different message for if it wasn't successful.

Thanks - I'm still a rookie!

EDIT: so after using some code from the answer i am now at:

$username = $_POST["username"];
$password_input = $_POST["password"];
$password = md5($password_input);
$email_input = $_POST["emailaddress"];
$email = md5($email_input);

if (!($stmt  = $con->prepare("INSERT INTO `users` (`username`,`password`,`email_address`) VALUES (?,?,?)")) || !is_object($stmt)) {die( "Error preparing: (" .$con->errno . ") " . $con->error);}
$stmt->bind_param('sss', $username, $password, $email);
$stmt->execute();
$stmt->close();
if ($con->affected_rows == 1) {echo "User has been Created! Feel free to login - <a href='login.php'><span class='button color_blue'>Login</span></a>";}
var_dump($con->affected_rows);

successful and unsuccessful INSERTS for some reason all have -1 as their "affected rows" output

The execute() method returns true if successful.

Replace:

$stmt->execute();

with:

if($stmt->execute()) { 
    echo "user created!";
} else {
    echo "error: " . $stmt->error;
}

To check for successful UPDATE/INSERT/DELETE query, you would need to check if returned affected rows are more than zero.

http://php.net/mysqli_affected_rows

Returns the number of rows affected by the last INSERT, UPDATE, REPLACE or DELETE query.

In your case:

if ($con->affected_rows == 1) {
    echo "User has been Created! Feel free to login - <a href='login.php'><span class='button color_blue'>Login</span></a>";
}

You could use mysqli_stmt::affected_rows to find out if rows were affected. If not, you can print out your error message

Yes, you just have to check if the query was successful or not:

change the end of your code by this:

$success = $stmt->execute();
$stmt->close();
if ($success)
  echo "User has been Created! Feel free to login - <a href='login.php'><span class='button color_blue'>Login</span></a>";
else
  echo "Impossible to create that user: ".$stmt->error;

To provide many different messages I work with flags. Simply say: if the username equals an existing db name, userExistMsg = 1. To check if the User Exists, simply use a SELECT query in sql and ask for any entrys on the given user. This query should result in none obkects if the username is free.

Btw. You seem to use simple md5 for pw encoding. That's not verry secure. Better use something like salted passwords.

Try this. This is a sample answer change this according to your code..

     $username = $_POST["username"];
        $password_input = $_POST["password"];
        $password = md5($password_input);
        $email_input = $_POST["emailaddress"];
        $email = md5($email_input);

        $con=mysqli_connect("HOST","USER","PASSWORD","your_db");
        // Check connection
        if (mysqli_connect_errno())
          {
          echo "Failed to connect to MySQL: " . mysqli_connect_error();
          }

        $result = mysqli_query($con,"SELECT COUNT(username) FROM users WHERE username = $username ");

      $datacount2 = mysql_num_rows($result );  
        if($datacount2 < 1)
        {
        if (!($stmt  = $con->prepare("INSERT INTO `users` (`username`,`password`,`email_address`) VALUES (?,?,?)")) || !is_object($stmt)) {
            die( "Error preparing: (" .$con->errno . ") " . $con->error);
        }
        $stmt->bind_param('sss', $username, $password, $email);
        $stmt->execute();
        $stmt->close();

    if($stmt)
     {
        echo "User has been Created! Feel free to login - <a href='login.php'><span class='button color_blue'>Login</span></a>";
      }
      else
     {
     echo "Insert Failed";
     }
        }

        else
        {
        echo "User already exists..";
        }