Zero to SOLID

Zero to SOLID
in 45 Minutes
by Vic Metcalfe
@v_metcalfe

I made an
e-Commerce
site in PHP!

Children or those feint of
heart are warned to leave
the room…

<?php
session_start();
$connection = new PDO('mysql:host=localhost;dbname=solid', 'root', '');
if (!isset($_SESSION['cart'])) {
$connection->exec("INSERT INTO cart () VALUES ()");
$_SESSION['cart'] = $connection->lastInsertId();
}
if (isset($_POST['addproduct'])) {
$sql = "INSERT INTO cartitem (cart, product, quantity)
VALUES (:cart, :product, :quantity)
ON DUPLICATE KEY UPDATE quantity = quantity + :quantity";
$parameters = [
'cart' => $_SESSION['cart'],
'product' => $_POST['addproduct'],
'quantity' => $_POST['quantity'],
];
$statement = $connection->prepare($sql);
$statement->execute($parameters);
}
if (isset($_POST['update'])) {
$sql = "UPDATE cartitem SET quantity=:quantity
WHERE cart=:cart and product=:product";
$parameters = [
'product' => $_POST['update'],
];
}
https://github.com/zymsys/solid/blob/00/cart.php

$statement = $connection->prepare("SELECT * FROM cartitem
WHERE cart=:cart AND quantity <> 0");
$statement->execute(['cart' => $_SESSION['cart']]);
$cartItems = $statement->fetchAll();
?>
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>GTA-PHP Gift Shop</title>
<link rel="stylesheet" href="site.css">
</head>
<body>
<div class="container">
<h1>GTA-PHP Gift Shop</h1>
<p>Buy our junk to keep our organizers up to date
with the latest gadgets.</p>
<table class="table">
<tr>
<th>Product Name</th>
<th>You Pay</th>
<th>Group Gets</th>
<th></th>
</tr>
<?php
$products = [];
$result = $connection->query("SELECT * FROM product");
foreach ($result as $product) {
$products[$product['id']] = $product;
?>
<tr>
<td><?php echo $product['name']; ?></td>

<td><?php
$price = $product['price'];
echo number_format($price / 100, 2);
?></td>
<td>
<?php
echo number_format(
($product['price'] - $product['cost']) / 100, 2
);
?>
</td>
<td>
<form method="post">
<input type="number" name="quantity"
value="1" style="width: 3em">
<input type="hidden" name="addproduct"
value="<?php echo $product['id']; ?>">
<input class="btn btn-default btn-xs"
type="submit" value="Add to Cart">
</form>
</td>
</tr>
<?php
}
?>
</table>
<?php if (count($cartItems) > 0): ?>
<?php
$total = 0;
$taxable = 0;
$provinceCode = isset($_GET['province']) ?
$_GET['province'] : 'ON'; //Default to GTA-PHP's home
$provinces = [];

$result = $connection->query("SELECT * FROM province
ORDER BY name");
foreach ($result as $row) {
$provinces[$row['code']] = $row;
if ($row['code'] === $provinceCode) {
$province = $row;
}
}
?>
<h2>Your Cart:</h2>
<?php foreach ($cartItems as $cartItem): ?>
<?php $product = $products[$cartItem['product']]; ?>
<tr>
<td>
<?php echo $product['name']; ?>
</td>
<td>
Quantity:
<input type="hidden" name="update"
<input type="number" name="quantity" style="width: 3em"
value="<?php echo $cartItem['quantity']; ?>">
<button type="submit">Update</button>
</form>
</td>
<td>
<?php
echo number_format(
$cartItem['quantity'] * $product['price'] / 100, 2
);
$itemTotal = $cartItem['quantity'] * $product['price'];

$total += $itemTotal;
$taxable += $product['taxes'] ? $itemTotal : 0;
?>
</td>
</tr>
<?php endforeach; ?>
<tr>
<td></td>
<td style="text-align: right">Subtotal:</td>
<td><?php echo number_format($total / 100, 2); ?></td>
</tr>
<tr>
<td></td>
<td style="text-align: right">
<?php echo $province['name']; ?> taxes at
<?php echo $province['taxrate'] ?>%:</td>
<td>
<?php
$taxes = $taxable * $province['taxrate'] / 100;
$total += $taxes;
echo number_format($taxes / 100, 2);
?>
</td>
</tr>
<tr>
<td></td>
<td style="text-align: right">Total:</td>
<td><?php echo number_format($total / 100, 2); ?></td>
</tr>
</table>
<form method="get">
Calculate taxes for purchase from:
<select name="province">

<?php foreach ($provinces as $province): ?>
<?php
$selected = $provinceCode === $province['code'] ? 'selected' : '';
?>
<option value="<?php echo $province['code']; ?>"
<?php echo $selected; ?>>
<?php echo $province['name']; ?>
</option>
</select>
<button type="submit" class="btn btn-default btn-xs">
Recalculate</button>
</form>
<form action="checkout.php" method="post">
<?php foreach ($cartItems as $itemNumber => $cartItem): ?>
<?php
$product = $products[$cartItem['product']];
?>
<input type="hidden" name="item<?php echo $itemNumber; ?>"
value="<?php echo $product['name'] . '|' .
number_format($product['price'] / 100, 2); ?>">
<input type="hidden" name="item<?php echo count($cartItems); ?>"
value="<?php echo 'Tax|' . number_format($taxes / 100, 2); ?>">
<button type="submit" class="btn btn-primary" style="float: right">
Checkout
</button>
</form>
<?php endif; ?>
</div>
</body>
</html>

Is there anything
wrong with this?

Step 1:
Separate PHP from
HTML

<?php
function initialize()
{
global $connection;
session_start();
$connection = new PDO('mysql:host=localhost;dbname=solid', 'root', '');
$connection->exec("INSERT INTO cart () VALUES ()");
$_SESSION['cart'] = $connection->lastInsertId();
}
}
function handleAdd()
{
global $connection;
if (!isset($_POST['addproduct'])) {
return;
}
$parameters = [
'product' => $_POST['addproduct'],
];
}

function loadProducts()
{
global $connection;
$products = [];
$result = $connection->query("SELECT * FROM product");
}
return $products;
}
function loadProvinces()
{
global $connection;
$provinces = [];
$result = $connection->query("SELECT * FROM province ORDER BY name");
}
return $provinces;
}

<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>GTA-PHP Gift Shop</title>
<link rel="stylesheet" href="site.css">
</head>
<body>
<div class="container">
<h1>GTA-PHP Gift Shop</h1>
<p>Buy our junk to keep our organizers up to date
with the latest gadgets.</p>
<tr>
<th>Product Name</th>
<th>You Pay</th>
<th>Group Gets</th>
<th></th>
</tr>
<?php foreach ($viewData['products'] as $product): ?>
<tr>
<td><?php echo $product['name']; ?></td>
<td><?php
$price = $product['price'];
echo number_format($price / 100, 2);
?></td>
<td><?php
echo number_format(
($product['price'] - $product['cost']) / 100, 2
);
?></td>

<td>
<input type="number" name="quantity"
value="1" style="width: 3em">
<input type="hidden" name="addproduct"
<input class="btn btn-default btn-xs"
type="submit" value="Add to Cart">
</form>
</td>
</tr>
</table>
<?php if (count($viewData['cartItems']) > 0): ?>
<h2>Your Cart:</h2>
<?php foreach ($viewData['cartItems'] as $cartItem): ?>
<?php $product = $viewData['products'][$cartItem['product']]; ?>
<tr>
<td>
<?php echo $product['name']; ?>
</td>
<td>
Quantity:
<input type="hidden" name="update"
<input type="number" name="quantity" style="width: 3em"
value="<?php echo $cartItem['quantity']; ?>">
<button type="submit">Update</button>
</form>
</td>

<td>
<?php
echo number_format(
$cartItem['quantity'] * $product['price'] / 100, 2
);
?>
</td>
</tr>
<tr>
<td></td>
<td style="text-align: right">Subtotal:</td>
<td><?php echo number_format($viewData['subtotal'] / 100, 2); ?></td>
</tr>
<tr>
<td></td>
<?php echo $viewData['province']['name']; ?> taxes at
<?php echo $viewData['province']['taxrate'] ?>%:</td>
<td>
<?php
echo number_format($viewData['taxes'] / 100, 2);
?>
</td>
</tr>
<tr>
<td></td>
<td style="text-align: right">Total:</td>
<td><?php echo number_format($viewData['total'] / 100, 2); ?></td>
</tr>
</table>

<form method="get">
Calculate taxes for purchase from:
<select name="province">
<?php foreach ($viewData['provinces'] as $province): ?>
<?php
$selected = $viewData['provinceCode'] ===
$province['code'] ? 'selected' : '';
?>
<option value="<?php echo $province['code']; ?>"
<?php echo $selected; ?>>
<?php echo $province['name']; ?>
</option>
</select>
<button type="submit" class="btn btn-default btn-xs">Recalculate</button>
</form>
<form action="checkout.php" method="post">
<?php foreach ($viewData['cartItems'] as $itemNumber => $cartItem): ?>
<?php
$product = $viewData['products'][$cartItem['product']];
?>
<input type="hidden" name="item<?php echo $itemNumber; ?>"
value="<?php
echo $product['name'] . '|' .
number_format($product['price'] / 100, 2);
?>">

<input type="hidden"
name="item<?php echo count($viewData['cartItems']); ?>"
value="<?php echo 'Tax|' .
number_format($viewData['taxes'] / 100, 2); ?>">
<button type="submit" class="btn btn-primary" style="float: right">
Checkout</button>
</form>
<?php endif; ?>
</div>
</body>
</html>

Objects
A very brief introduction

Objects help us to
organize our code

How might we group these functions?

Invisible stuff
that happens on
page load
Loads stuff into
our HTML (view)

Objects help us to
encapsulate code

private
private
public
private
private
public
private
private
private

class Initializer
{
private $connection;
public function __construct(PDO $connection)
{
$this->connection = $connection;
}
public function initialize()
{
session_start();
$this->connection->exec("INSERT INTO cart () VALUES ()");
$_SESSION['cart'] = $this->connection->lastInsertId();
}
$this->handleAdd();
$this->handleUpdate();
}
private function handleAdd() { … }
private function handleUpdate() { … }
}

class ViewData {
{
}
private function loadCartItems() { … }
private function loadProducts(){ … }
private function loadProvinces(){ … }
private function calculateCartSubtotal($cartItems, $products) { … }
private function calculateCartTaxes($cartItems, $products, $taxrate) { … }
public function buildViewData() { … }
}

public function buildViewData()
{
$viewData = [
'cartItems' => $this->loadCartItems(),
'products' => $this->loadProducts(),
'provinces' => $this->loadProvinces(),
];
}
}
$viewData['subtotal'] = $this->calculateCartSubtotal($viewData['cartItems'],
$viewData['products']);
$viewData['taxes'] = $this->calculateCartTaxes($viewData['cartItems'],
return $viewData;
}

SOLID
Fewer Gremlins in your Code

SRP
Single Responsibility Principal

Business Responsibility
not code

Sales
Accounting
Inventory
Application / IT

class Application
{
public function __construct()
{
$this->connection = new PDO('mysql:host=localhost;dbname=solid', 'root', '');
$this->inventory = new Inventory($this->connection);
$this->sales = new Sales($this->connection);
$this->accounting = new Accounting($this->connection);
}
{
session_start();
$this->connection->exec("INSERT INTO cart VALUES ()");
}
$this->handlePost();
}

{
$viewData = [
'cartItems' => $this->sales->loadCartItems(),
'products' => $this->inventory->loadProducts(),
'provinces' => $this->accounting->loadProvinces(),
];
}
}
$viewData['subtotal'] = $this->accounting->
calculateCartSubtotal($viewData['cartItems'], $viewData['products']);
$viewData['taxes'] = $this->accounting->
calculateCartTaxes($viewData['cartItems'],
return $viewData;
}
//Class Application Continued…

private function handlePost()
{
if (isset($_POST['addproduct'])) {
$this->sales->addProductToCart(
$_SESSION['cart'],
$_POST['addproduct'],
$_POST['quantity']
);
}
if (isset($_POST['update'])) {
$this->sales->modifyProductQuantityInCart(
$_SESSION['cart'],
$_POST['update'],
$_POST['quantity']
);
}
}
}
//Class Application Continued…

class Inventory {
{
}
public function loadProducts()
{
$products = [];
$result = $this->connection->query("SELECT * FROM product");
}
return $products;
}
}

class Sales {
{
}
public function addProductToCart($cartId, $productId, $quantity)
{
$parameters = [
'cart' => $cartId,
'product' => $productId,
'quantity' => $quantity,
];
$statement = $this->connection->prepare($sql);
}

public function modifyProductQuantityInCart($cartId, $productId, $quantity)
{
$parameters = [
'cart' => $cartId,
'product' => $productId,
'quantity' => $quantity,
];
$statement = $this->connection->prepare($sql);
}
public function loadCartItems()
{
$statement = $this->connection->prepare("SELECT * FROM cartitem
return $statement->fetchAll();
}
}
//Class Sales Continued…

class Accounting {
{
}
public function loadProvinces()
{
$provinces = [];
$result = $this->connection->query("SELECT * FROM province ORDER BY name");
}
return $provinces;
}
public function calculateCartSubtotal($cartItems, $products)
{
$subtotal = 0;
$subtotal += $cartItem['quantity'] * $product['price'];
}
return $subtotal;
}

public function calculateCartTaxes($cartItems, $products, $taxrate)
{
$taxable = 0;
}
return $taxable * $taxrate / 100;
}
}
//Class Accounting Continued…

Open to Extension
Closed to Modification

New requirement:
10% off orders over $100

First we need to
understand inheritance
and polymorphism

Classes can extend
other classes

class AccountingStrategy {
private $description;
public function __construct($description)
{
$this->description = $description;
}
public function getAdjustment($cartItems)
{
return false;
}
public function getDescription()
{
return $this->description;
}
}

class TaxAccountingStrategy extends AccountingStrategy {
private $products;
private $taxRate;
public function __construct($products, $province)
{
parent::__construct($province['name'] . ' taxes at ' .
$province['taxrate'] . '%:');
$this->products = $products;
$this->taxRate = $province['taxrate'];
}
{
$taxable = 0;
$product = $this->products[$cartItem['product']];
}
return $taxable * $this->taxRate / 100;
}
}

TaxAccountingStrategy is
an AccountingStrategy.

{
session_start();
}
$this->products = $this->inventory->loadProducts();
$provinceRepository = new ProvinceRepository($this->connection,
isset($_GET['province']) ? $_GET['province'] : 'ON');
$this->provinces = $provinceRepository->loadProvinces();
$this->selectedProvince = $provinceRepository->getSelectedProvince();
$this->accounting->addStrategy(
new TaxAccountingStrategy(
$this->products,
$provinceRepository->getSelectedProvince()
)
);
}
{
session_start();
}
}

{
$viewData = [
'cartItems' => $this->sales->loadCartItems(),
'products' => $this->inventory->loadProducts(),
'provinces' => $this->accounting->loadProvinces(),
];
{
$cartItems = $this->sales->loadCartItems();
$viewData = [
'cartItems' => $cartItems,
'products' => $this->products,
'provinces' => $this->provinces,
'adjustments' => $this->accounting->applyAdjustments($cartItems),
'provinceCode' => $this->selectedProvince['code'],
];
Done in
initialize() now
Used Twice
New!
Start of buildViewData()

End of buildViewData()
$viewData['taxes'] = $this->accounting->
calculateCartTaxes($viewData['cartItems'],
return $viewData;
}
$viewData['total'] = $viewData['subtotal'] +
$this->accounting->getAppliedAdjustmentsTotal();
return $viewData;
}
Taxes are handled by adjustments
and removed as a specific item in
the view’s data.

// loadProvinces used to live in the Accounting class
class ProvinceRepository
{
private $provinces = null;
private $selectedProvince;
private $selectedProvinceCode;
public function __construct(PDO $connection, $selectedProvinceCode)
{
$this->selectedProvinceCode = $selectedProvinceCode;
}
public function loadProvinces() { … } // Now sets $selectedProvince
public function getProvinces()
{
return is_null($this->provinces) ? $this->loadProvinces() : $this->provinces;
}
public function getSelectedProvince()
{
return $this->selectedProvince;
}
}

Remove calculateCartTaxes and add
AccountingStrategy
class Accounting {
private $strategies = [];
private $appliedAdjustments = 0;
{
}
public function calculateCartSubtotal($cartItems, $products) { … } // No change
public function addStrategy(AccountingStrategy $strategy)
{
$this->strategies[] = $strategy;
}

public function applyAdjustments($cartItems)
{
$adjustments = [];
foreach ($this->strategies as $strategy) {
$adjustment = $strategy->getAdjustment($cartItems);
if ($adjustment) {
$this->appliedAdjustments += $adjustment;
$adjustments[] = [
'description' => $strategy->getDescription(),
'adjustment' => $adjustment,
];
}
}
return $adjustments;
}
public function getAppliedAdjustmentsTotal()
{
return $this->appliedAdjustments;
}
}
//Class Accounting Continued…

<?php foreach ($viewData['adjustments'] as $adjustment): ?>
<tr>
<td></td>
<?php echo $adjustment['description']; ?>
</td>
<td>
<?php
echo number_format($adjustment['adjustment'] / 100, 2);
?>
</td>
</tr>

<?php
for ($adjustmentIndex = 0;
$adjustmentIndex < count($viewData['adjustments']);
$adjustmentIndex += 1):
$adjustment = $viewData['adjustments'][$adjustmentIndex];
?>
<input type="hidden"
name="item<?php echo $adjustmentIndex; ?>"
value="<?php echo $adjustment['description'] . '|' .
number_format($adjustment['adjustment'] / 100, 2); ?>">
<?php endfor; ?>

Now we can add
DiscountAccountingStrategy
without modifying Accounting or
the view

class DiscountAccountingStrategy extends AccountingStrategy {
private $products;
public function __construct($products)
{
parent::__construct("Discount for orders over $100");
$this->products = $products;
}
{
$total = array_reduce($cartItems, function ($carry, $item) {
$product = $this->products[$item['product']];
return $carry + $item['quantity'] * $product['price'];
}, 0);
return $total > 10000 ? ($total / -10) : false;
}
}

In Application::initialize()
$this->accounting->addStrategy(
new DiscountAccountingStrategy($this->products)
);

LSP
Liskov Substitution Principal

If a class is a thing, it
should act like that thing.
must

Ways to break LSP
• Throw a new exception
• Add requirements to parameters
• requiring a more specific type
• Return an unexpected type
• returning a less specific type
• Do anything that would be unexpected if used as a
stand-in for an ancestor

DIP
Dependency Inversion Principle

Yeah smarty-pants, the D
does come before the I

Classes can implement
interfaces, and can
depend on interfaces

We can scrap the
AccountingStrategy class and add…
interface AccountingStrategyInterface
{
public function getDescription();
public function getAdjustment($cartItems);
}

Because Application creates concrete
classes there’s little to be gained by
adding other interfaces
public function __construct()
{
$this->connection = new PDO('mysql:host=localhost;dbname=solid', 'root', '');
$this->inventory = new Inventory($this->connection);
$this->sales = new Sales($this->connection);
$this->accounting = new Accounting($this->connection);
}
Dependency Injection Containers would
solve this, but are a topic for another talk.

ISP
Interface Segregation Principal

If we follow SRP and
interfaces describe classes,
what’s left to segregate?

Imagine an interface to our
Sales class
interface SalesInterface
{
public function addProductToCart($cartId, $productId, $quantity);
public function modifyProductQuantityInCart($cartId, $productId, $quantity);
public function loadCartItems();
}

Imagine an interface to our
Sales class
interface SalesWriterInterface
{
public function addProductToCart($cartId, $productId, $quantity);
public function modifyProductQuantityInCart($cartId, $productId, $quantity);
public function loadCartItems();
}
interface SalesReaderInterface
{
}

Last words
Resist Overengineering
Simplify don’t complicate
Pragmatic not Dogmatic
Questions?

Zero to SOLID

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