How is the variable scope of a PHP function determined?

The variable scope in PHP is divided into local (within the function), global (accessible within the program), and class scope (accessible within the class instance). The global keyword can declare local variables as global variables, and the static keyword can declare local variables as static variables, retaining their values ​​between function calls.

Determination of variable scope in PHP

In PHP, the scope of a variable refers to which parts of the code can access and modify the variable. Understanding variable scope is crucial to writing robust and maintainable PHP code.

Scope rules of PHP

• Local scope: Variables defined inside a function or method can only be accessed inside that function or method.
• Global scope: Variables defined in the global scope can be accessed anywhere in the program.
• Class scope: Constants and static variables defined inside a class but not in a function or method have class scope and can be accessed within instances of the class.

Keywords that affect variable scope

• global: Declare local variables as global variables, allowing global variables to be accessed inside the function.
• static: Declare a local variable as a static variable so that it retains its value between function calls.

// 全局变量
$global_var = 10;

function test() {
    // 局部变量
    $local_var = 5;

    // 使用全局变量，需要使用 global 关键字
    global $global_var;

    // 使用静态变量
    static $static_var = 0;
    
    $static_var++;
    
    // 输出变量值
    echo "局部变量：$local_var <br>";
    echo "全局变量：$global_var <br>";
    echo "静态变量：$static_var <br>";
}

test(); // 输出： 局部变量：5 <br> 全局变量：10 <br> 静态变量：1
test(); // 输出： 局部变量：5 <br> 全局变量：10 <br> 静态变量：2

Practical Case

Suppose there is a shopping cart class used to manage items in shopping. This class contains a private variable named $items that stores the list of items in the shopping cart.

class Cart {
    private $items = [];

    public function addItem($item) {
        $this->items[] = $item;
    }

    public function getItems() {
        return $this->items;
    }
}

// 创建购物车实例
$cart = new Cart;

// 添加商品
$cart->addItem('商品 1');
$cart->addItem('商品 2');

// 获取购物车中的商品
$items = $cart->getItems();

// 输出购物车中商品, 使用循环遍历或打印数组
foreach ($items as $item) {
    echo $item . '<br>';
}

In this example, the $items variable has a private scope and can only be accessed and modified within the instance method of the Cart class. This variable is available throughout the life cycle of the class instance, realizing the encapsulation and management of the items in the shopping cart.

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