Error Message not working for PHP user check - php

Question

I am new to PHP and have wrote a register PHP page, it is working fine and can add to database for new users. But when adding repeated username, the error message does not appear. did i miss out any codes or do i need to change the codes?
My Code:
<?php
define('DB_HOST', 'localhost');
define('DB_NAME', 'login');
define('DB_USER','root');
define('DB_PASSWORD','');
$con=mysql_connect(DB_HOST,DB_USER,DB_PASSWORD) or die("Failed to connect to MySQL: " . mysql_error());
$db=mysql_select_db(DB_NAME,$con) or die("Failed to connect to MySQL: " . mysql_error());
function NewUser()
{
$username = $_POST['username'];
$password = $_POST['password'];
$firstname = $_POST['firstname'];
$surname = $_POST['surname'];
$query = "INSERT INTO users (username, password, firstname, surname) VALUES ('$username', '$password', '$firstname', '$surname')";
$data = mysql_query ($query)or die(mysql_error());
if($data)
{
echo "YOUR REGISTRATION IS COMPLETED, YOU CAN SIGN IN NOW";
}
}
function SignUp()
{
if(!empty($_POST['username']))
{
$query = mysql_query("SELECT * FROM users WHERE username = '$_POST[username]' AND password = '$_POST[password]'") or die(mysql_error());
if(!$row = mysql_fetch_array($query) or die(mysql_error()))
{
newuser();
}
else
{
echo "SORRY, YOU ARE ALREADY REGISTERED USER.";
}
}
}
if(isset($_POST['submit']))
{
SignUp();
}
?>

Answer 1

Since you are new, you might as well ditch the MySQL_ statements and go with something more secure like MySQLi_ or PDO. This won't disallow you to use databases, they are just more secure.
Also: $row is undefined.
And once again, because you are new, I suggest using an editor that gives you indentation to your code. Indentation makes your code more readable, and helps other people help you when you have problems like this.

Answer 2

$row is undefined and try using mysql_num_rows
when ever you want to access the column values you can use mysql_fetch_array
here in this case you just checking for $query returned a row or not.
so you can change you code to
as you did it for
$data = mysql_query ($query)or die(mysql_error());
if($data)
{
echo "YOUR REGISTRATION IS COMPLETED, YOU CAN SIGN IN NOW";
}
$query = mysql_query("SELECT * FROM users WHERE username = '$_POST[username]' AND password = '$_POST[password]'") or die(mysql_error());
if($query){
newuser();
}else{
echo "SORRY, YOU ARE ALREADY REGISTERED USER.";
}