<?php
$username = $password = $confirm_password = "";
$username_err = $password_err = $confirm_password_err = "";

if($_SERVER["REQUEST_METHOD"] == "POST"){

    if(empty(trim($_POST["uname"]))){
        //Vul een gebruikersnaam in error!
    } else{
        $sql = "SELECT id FROM users WHERE username = ?";
        if($stmt = $mysqli->prepare($sql)){

            $stmt->bind_param("s", $param_username);
            $param_username = trim($_POST["uname"]);

            if($stmt->execute()){
                $stmt->store_result();
                if($stmt->num_rows == 1){
                    //Gebruikersnaam al bezet error!
                } else{
                    $username = trim($_POST["uname"]);
                }
            } else{
                //MYSQL fout!
            }
        }
        $stmt->close();
    }
    if(empty(trim($_POST['pwd']))){
        //Vul een wachtwoord in error
    } elseif(strlen(trim($_POST['pwd'])) < 6){
        //Minimaal 6 tekens error
    } else{
        $password = trim($_POST['pwd']);
    }

    if(empty($username_err) && empty($password_err) && empty($confirm_password_err)){

        $sql = "INSERT INTO users (username, password) VALUES (?, ?)";
        if($stmt = $mysqli->prepare($sql)){
            $stmt->bind_param("ss", $param_username, $param_password);
            $param_username = $username;
            $param_password = password_hash($password, PASSWORD_DEFAULT);
            if($stmt->execute()){
                //Redirect naar login! Daar een session opmaken!
            } else{
                //MYSQL error
            }
        }
        $stmt->close();
    }
    $mysqli->close();
}
?>
<form action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]); ?>" method="post">
  <table>
      <tr>
        <td>Username:</td>
        <td><input type="text" name="uname" placeholder="Username" /></td>
      </tr>
      <tr>
        <td>Password:</td>
        <td><input type="password" name="pwd" placeholder="Password" /></td>
      </tr>
      <tr>
        <td colspan="2"><input type="submit" name="submit" value="Login" /></td>
      </tr>
    </table>
</form>