Untitled

<?php
$con = mysql_connect("localhost","root","");
if (!$con)
  {
  die('Could not connect: ' . mysql_error());
  }

mysql_select_db("users", $con) or die("Can't select db.");

$username = $_POST['username']; // or $_POST["username"]; Doesn't matter if " or ' here
$password = $_POST['password'];
$error = false;

if($_POST){
if(empty($username){ //$username is exactly the same as $_POST['username']
   echo "Empty username!";
   $error = true;
}

if(empty($password)){
   echo "Empty password";
   $error = true;
}

if($error != true){ //Or if($error == false){ - it's the same
$usercheck = mysql_query("SELECT * FROM users WHERE username='$username' LIMIT 1");

if(mysql_num_rows($usercheck) == 0){
mysql_query("INSERT INTO users (username, password)
VALUES ('$username', '$password')") or die(mysql_error());
echo "Registration Succesful";
} else {
echo "Username {$username} is already taken.";
}
}
}

mysql_close($con);
?>


<!DOCTYPE html>
<html>
<body>

<form action="register.php" method="POST">
Username: <input type="text" name="username"><br>
Password: <input type="password" name="password"><br />
<input type="submit" value="Login."></form> <br />


</body>
</html>