[Strengthening the PHP series] Shopping cart code explains PHP’s anonymous functions

【Strengthening the PHP Series】Shopping Cart Code Description of PHP's Anonymous Functions

1. Definition: Anonymous functions (Anonymous functions), also called closures, allow temporary creation of a function without a specified name. The value most commonly used as a callback function argument. Of course, there are other applications as well.

2. Usage:

　1) As the value of a variable:

　 The closure function can also be used as the value of a variable. PHP will automatically convert this expression into an object instance of the built-in class Closure. The method of assigning a closure object to a variable is the same as the syntax of ordinary variable assignment, and a semicolon must be added at the end

　2) Inherit variables from the parent scope:

　 Closures can inherit variables from the parent scope . Any such variables should be passed in using the use language construct.

　3) A complete example, illustrated with shopping cart code:

<?php
// 一个基本的购物车，包括一些已经添加的商品和每种商品的数量。
// 其中有一个方法用来计算购物车中所有商品的总价格，该方法使
// 用了一个 closure 作为回调函数。
class Cart
{
    const PRICE_BUTTER  = 1.00;
    const PRICE_MILK    = 3.00;
    const PRICE_EGGS    = 6.95;

    protected   $products = array();
    
    public function add($product, $quantity)
    {
        $this->products[$product] = $quantity;
    }
    
    public function getQuantity($product)
    {
        return isset($this->products[$product]) ? $this->products[$product] :
               FALSE;
    }
    
    public function getTotal($tax)
    {
        $total = 0.00;
        
        $callback =
            function ($quantity, $product) use ($tax, &$total)
            {
                $pricePerItem = constant(__CLASS__ . "::PRICE_" .
                    strtoupper($product));
                $total += ($pricePerItem * $quantity) * ($tax + 1.0);
            };
        
        array_walk($this->products, $callback);
        return round($total, 2);;
    }
}

$my_cart = new Cart;

// 往购物车里添加条目
$my_cart->add(&#39;butter&#39;, 1);
$my_cart->add(&#39;milk&#39;, 3);
$my_cart->add(&#39;eggs&#39;, 6);

// 打出出总价格，其中有 5% 的销售税.
print $my_cart->getTotal(0.05) . "\n";
// 最后结果是 54.29
?>

3. Reference:

　1) PHP official description of 'anonymous function': http://www.php.net/manual/zh /functions.anonymous.php

　2) The constant() function returns a constant value: http://www.runoob.com/php/func-misc-constant.html

　3) The array_walk() function returns a constant value in an array Apply a user-defined function to each element: http://www.w3school.com.cn/php/func_array_walk.asp

　　4) The round() function rounds floating point numbers: http://www.w3school.com .cn/php/func_math_round.asp