PHP Object-oriented programming design patterns have always been a hot topic studied by developers. In this article, PHP editor Strawberry will delve into the SOLID principles to help readers understand and apply these design principles. The SOLID principle is the cornerstone of object-oriented programming, including the single responsibility principle, the open and closed principle, the Liskov substitution principle, the interface isolation principle and the dependency inversion principle. By learning and practicing these principles, developers can write more flexible, maintainable, and scalable PHP code.

1. Single Responsibility Principle (SRP): A class should be responsible for only one task, and this task should be encapsulated in the class. This can improve the maintainability and reusability of the class.

class User {
private $id;
private $name;
private $email;

public function __construct($id, $name, $email) {
$this->id = $id;
$this->name = $name;
$this->email = $email;
}

public function getId() {
return $this->id;
}

public function getName() {
return $this->name;
}

public function getEmail() {
return $this->email;
}
}

class UserRepository {
private $connection;

public function __construct($connection) {
$this->connection = $connection;
}

public function save(User $user) {
$stmt = $this->connection->prepare("INSERT INTO users (name, email) VALUES (?, ?)");
$stmt->execute([$user->getName(), $user->getEmail()]);
}

public function find($id) {
$stmt = $this->connection->prepare("SELECT * FROM users WHERE id = ?");
$stmt->execute([$id]);
$result = $stmt->fetch();

if ($result) {
return new User($result["id"], $result["name"], $result["email"]);
}

return null;
}
}

2. Open-Closed Principle (OCP): Software entities (classes, modules, etc.) should be open to extensions and closed to modifications. This can improve the flexibility of the software and reduce software maintenance costs.

interface Shape {
public function getArea();
}

class Rectangle implements Shape {
private $width;
private $height;

public function __construct($width, $height) {
$this->width = $width;
$this->height = $height;
}

public function getArea() {
return $this->width * $this->height;
}
}

class Circle implements Shape {
private $radius;

public function __construct($radius) {
$this->radius = $radius;
}

public function getArea() {
return pi() * $this->radius ** 2;
}
}

class ShapeCalculator {
public function calculateTotalArea($shapes) {
$totalArea = 0;
foreach ($shapes as $shape) {
$totalArea += $shape->getArea();
}

return $totalArea;
}
}

3. Liskov Substitution Principle (LSP): A subclass can replace its parent class without affecting the correctness of the program. This increases the flexibility of the software and makes it easier to refactor.

class Animal {
public function eat() {
echo "Animal is eating.";
}
}

class Dog extends Animal {
public function bark() {
echo "Dog is barking.";
}
}

class Cat extends Animal {
public function meow() {
echo "Cat is meowing.";
}
}

function feedAnimal(Animal $animal) {
$animal->eat();
}

feedAnimal(new Dog()); // prints "Dog is eating."
feedAnimal(new Cat()); // prints "Cat is eating."

4. Interface Segregation Principle (ISP): Multiple specialized interfaces should be used instead of one universal interface. This can improve the readability of the software and reduce software maintenance costs.

interface Printable {
public function print();
}

interface Scannable {
public function scan();
}

interface Faxable {
public function fax();
}

class Printer implements Printable {
public function print() {
echo "Printer is printing.";
}
}

class Scanner implements Scannable {
public function scan() {
echo "Scanner is scanning.";
}
}

class FaxMachine implements Faxable {
public function fax() {
echo "Fax machine is faxing.";
}
}

class MultifunctionDevice implements Printable, Scannable, Faxable {
public function print() {
echo "Multifunction device is printing.";
}

public function scan() {

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