register.php (Check username taken)

if (isset($_POST['register'])) {
    if (empty($_POST['username']) || empty($_POST['password']) || empty($_POST['repassword'])) {
        $error = "Please fill in all the information";
    }

    else {
        $query = mysql_query("SELECT username FROM login WHERE username='$username'");
        $num = mysql_num_rows($query);

        if ($num > 0) {
            $error = "Username already taken";
        }

        else {
            // Define $username and $password
            $username=$_POST['username'];
            $password=$_POST['password'];
            $repassword=$_POST['repassword'];

            // Create connection
            $conn = new mysqli($dbhost, $dbuser, $dbpass, $dbname);

            // Check connection
            if ($conn->connect_error) {
                die("Connection failed: " . $conn->connect_error);
            }

            // Checking Username Length
            if (strlen($username) < 4) {
                $error = "Short username are easy to guess. Try one with at least 4 characters.";
            }

            // Checking Password Length
            elseif (strlen($password) < 6) {
                $error =  "Short passwords are easy to guess. Try one with at least 12 characters.";
            }

            else {
                // Checking Re-enter Password
                if ($password != $repassword) {
                    $error = "These passwords don't match. Try again?";
                }

                else {
                    // Encryption Password
                    $enpassword = md5($password);

                    $sql = "INSERT INTO login (username, password) VALUES ('$username', '$enpassword')";

                    if ($conn->query($sql) === TRUE) {
                        header("location: index.php");
                    }
                    else {
                        echo "Error: " . $sql . "<br>" . $conn->error;
                    }

                    $conn->close();
                }
            }
        }

    }
}