PHP add field to existing table

I have a database of multiple users, I want to be able to add more columns to the table after they have registered.

$username = $_POST["username"];
$email = $_POST["email"];
$password = $_POST["password"];
$statement = mysqli_prepare($con, "INSERT INTO user (username, email, password) VALUES (?, ?, ?)");
mysqli_stmt_bind_param($statement, "sss", $username, $email, $password);
mysqli_stmt_execute($statement);

$response = array();
$response["success"] = true;  

print_r(json_encode($response));

Here it saves their username, email and password. How would I add for columns for that individuals table?