PHP面向对象编程深入理解

PHP面向对象编程深入理解

PHP的面向对象从PHP5开始走向成熟，到了PHP8已经非常完善了。封装、继承、多态这三个基本特征要理解透彻，写出来的代码质量才会高。

先说说封装。封装就是把数据和操作数据的方法绑定在一起，对外隐藏内部实现细节。PHP通过访问控制修饰符来实现封装，public、protected、private分别控制不同级别的访问权限。

```php
class BankAccount
{
private string $accountNumber;
private float $balance;
private array $transactions = [];
private bool $frozen = false;

public function __construct(string $accountNumber, float $initialBalance = 0)
{
$this->accountNumber = $accountNumber;
$this->balance = $initialBalance;
$this->logTransaction('开户', $initialBalance);
}

public function deposit(float $amount): void
{
if ($this->frozen) {
throw new RuntimeException('账户已被冻结');
}
if ($amount <= 0) {
throw new InvalidArgumentException('存款金额必须大于0');
}

$this->balance += $amount;
$this->logTransaction('存款', $amount);
}

public function withdraw(float $amount): bool
{
if ($this->frozen) {
throw new RuntimeException('账户已被冻结');
}
if ($amount <= 0) {
throw new InvalidArgumentException('取款金额必须大于0');
}
if ($amount > $this->balance) {
return false;
}

$this->balance -= $amount;
$this->logTransaction('取款', -$amount);
return true;
}

public function getBalance(): float
{
return $this->balance;
}

public function getAccountNumber(): string
{
// 只显示后4位
return '****' . substr($this->accountNumber, -4);
}

public function getTransactionHistory(int $limit = 10): array
{
return array_slice(array_reverse($this->transactions), 0, $limit);
}

public function freeze(): void
{
$this->frozen = true;
}

public function unfreeze(): void
{
$this->frozen = false;
}

private function logTransaction(string $type, float $amount): void
{
$this->transactions[] = [
'type' => $type,
'amount' => $amount,
'balance' => $this->balance,
'time' => date('Y-m-d H:i:s'),
];
}
}

$account = new BankAccount('6222021234567890', 1000);
$account->deposit(500);
$account->withdraw(200);
echo "余额: {$account->getBalance()}\n";
echo "账号: {$account->getAccountNumber()}\n";
print_r($account->getTransactionHistory());
?>
```

继承是代码复用的重要手段。子类可以继承父类的属性和方法，还能重写或扩展。

```php
class SavingsAccount extends BankAccount
{
private float $interestRate;

public function __construct(string $accountNumber, float $initialBalance = 0, float $interestRate = 0.03)
{
parent::__construct($accountNumber, $initialBalance);
$this->interestRate = $interestRate;
}

public function applyInterest(): void
{
$interest = $this->getBalance() * $this->interestRate;
if ($interest > 0) {
$this->deposit($interest);
echo "利息 {$interest} 已入账\n";
}
}

public function getInterestRate(): float
{
return $this->interestRate;
}
}

class CheckingAccount extends BankAccount
{
private float $overdraftLimit;

public function __construct(string $accountNumber, float $initialBalance = 0, float $overdraftLimit = 500)
{
parent::__construct($accountNumber, $initialBalance);
$this->overdraftLimit = $overdraftLimit;
}

public function withdraw(float $amount): bool
{
if ($amount > $this->getBalance() + $this->overdraftLimit) {
return false;
}
return parent::withdraw($amount);
}
}

$savings = new SavingsAccount('SAV001', 5000, 0.04);
$savings->deposit(1000);
$savings->applyInterest();
echo "储蓄账户余额: {$savings->getBalance()}\n";

$checking = new CheckingAccount('CHK001', 1000, 500);
$checking->withdraw(1200);
echo "支票账户余额: {$checking->getBalance()}\n";
?>
```

多态让不同类的对象可以通过相同的接口调用。PHP通过接口和抽象类实现多态。

```php
interface PaymentMethod
{
public function processPayment(float $amount): array;
public function refund(string $transactionId): array;
public function getName(): string;
}

class CreditCardPayment implements PaymentMethod
{
private string $cardNumber;

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

public function processPayment(float $amount): array
{
return [
'id' => 'CC-' . uniqid(),
'amount' => $amount,
'status' => 'approved',
'method' => '信用卡',
'last4' => substr($this->cardNumber, -4),
];
}

public function refund(string $transactionId): array
{
return ['id' => $transactionId, 'status' => 'refunded'];
}

public function getName(): string { return '信用卡支付'; }
}

class PayPalPayment implements PaymentMethod
{
private string $email;

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

public function processPayment(float $amount): array
{
return [
'id' => 'PP-' . uniqid(),
'amount' => $amount,
'status' => 'completed',
'method' => 'PayPal',
'email' => $this->email,
];
}

public function refund(string $transactionId): array
{
return ['id' => $transactionId, 'status' => 'completed'];
}

public function getName(): string { return 'PayPal支付'; }
}

class WeChatPayment implements PaymentMethod
{
public function processPayment(float $amount): array
{
return [
'id' => 'WX-' . uniqid(),
'amount' => $amount,
'status' => 'success',
'method' => '微信支付',
];
}

public function refund(string $transactionId): array
{
return ['id' => $transactionId, 'status' => 'success'];
}

public function getName(): string { return '微信支付'; }
}

class OrderProcessor
{
public function processOrder(float $amount, PaymentMethod $payment): void
{
echo "使用 {$payment->getName()} 处理 {$amount} 元的订单\n";
$result = $payment->processPayment($amount);
echo "支付结果: {$result['status']}\n";
}
}

$processor = new OrderProcessor();
$processor->processOrder(99.99, new CreditCardPayment('6222123456789012'));
$processor->processOrder(199.99, new PayPalPayment('user@example.com'));
$processor->processOrder(299.99, new WeChatPayment());
?>
```

PHP8引入了一些新的OOP特性，让代码写起来更方便了。构造器属性提升、联合类型、match表达式、命名参数等等。

```php
// PHP8特性
class Order
{
public function __construct(
public readonly string $id,
public readonly string $userId,
public readonly array $items,
public readonly float $total,
public string $status = 'pending',
public ?string $paidAt = null,
) {}

public function markAsPaid(): void
{
$this->status = 'paid';
$this->paidAt = date('Y-m-d H:i:s');
}

public function isPending(): bool
{
return $this->status === 'pending';
}
}

// match表达式
function getOrderStatusText(string $status): string
{
return match ($status) {
'pending' => '待支付',
'paid' => '已支付',
'shipped' => '已发货',
'delivered' => '已签收',
'cancelled' => '已取消',
default => '未知状态',
};
}

// 命名参数
$order = new Order(
id: 'ORD-' . uniqid(),
userId: 'USR-123',
items: [
['product' => 'PHP', 'quantity' => 1, 'price' => 99.99],
],
total: 99.99,
);
$order->markAsPaid();
echo "订单 {$order->id} 状态: " . getOrderStatusText($order->status) . "\n";
?>
```

面向对象不是银弹。有些场景用函数式或者面向过程写反而更清晰。好的代码不在于用了什么范式，而在于可读性和可维护性。但在大型项目中，面向对象的组织方式确实有利于代码的管理和扩展。