PHP类实例调用中的未定义变量通知 [英] Undefined variable notices in PHP class instance call

问题描述

使用PHP，当我调用类的实例时，我得到的是未定义的变量通知。我使用其他语言编程，但是对PHP不太熟悉，能否请您查看我的代码并告知我定义类变量时的错误？

Using PHP, when I call an instance of a class I am getting an undefined variable notice back. I program in other languages, but am less familiar with PHP, can you please look at my code and inform me as to my mistake in defining my class variables? Thanks.

<?php
    // create class Bike
    class Bike
    {
        // create variables for class Bike of price, max_speed, and miles
        var $price;
        var $max_speed;
        var $miles;

        // create constructor for class Bike with user set price and max_speed
        function __construct($price, $max_speed)
        {
            $this->price = $price;
            $this->max_speed = $max_speed;
        }

        // METHODS for class Bike:
        // displayInfo() - have this method display the bike's price, maximum speed, and the total miles driven
        function displayInfo()
        {
            echo "Price: $" . $price .  "<p>Maximum Speed: " . $max_speed . "</p>" . "<p>Total Miles Driven: " . $miles . "</p>";
        }

        // drive() - have it display "Driving" on the screen and increase the total miles driven by 10
        function drive()
        {
            $miles = $miles + 10;
            echo "<p>Driving!</p>" . "<p>Total Miles Driven: " . $miles . "</p>";
        }

        // reverse() - have it display "Reversing" on the screen and decrease the total miles driven by 5...
        function reverse()
        {
            // What would you do to prevent the instance from having negative miles?
            if($miles >= 5)
            {
                $miles = $miles - 5;
                echo "<p>Reversing</p><p>Total Miles Driven: " . $miles . "</p>";
            }
            else
            {
                echo "<p>Total Miles Driven is less than 5 miles, unable to reverse!</p>";
            }
        }
    }
?>

<?php
    // Create 3 bike object class instances
    $Schwinn = new Bike(50, '10mph');
    $Specialized = new Bike(500, '25mph');
    $Cannondale = new Bike(1000, '50mph');

    // Have the first instance drive three times, reverse one and have it displayInfo().
    var_dump($Schwinn); // shows instance is created with proper variable assignments
    $Schwinn -> drive();
    // $Schwinn -> drive();
    // $Schwinn -> drive();
    $Schwinn -> reverse();
    $Schwinn -> displayInfo();
?>

错误：

通知：未定义变量：行27上/home/heidi/Desktop/CodingDojo/OOP/php_oop_basic1.php中的英里

注意：未定义变量：/ home / heidi / Desktop / CodingDojo /中的英里第35行的OOP / php_oop_basic1.php

注意：未定义的变量：第21行的/home/heidi/Desktop/CodingDojo/OOP/php_oop_basic1.php的价格

注意：未定义的变量：在第21行的/home/heidi/Desktop/CodingDojo/OOP/php_oop_basic1.php中的max_speed注意：未定义变量：在/home/heidi/Desktop/CodingDojo/OOP/php_oop_basic1.php中的英里21

Notice: Undefined variable: miles in /home/heidi/Desktop/CodingDojo/OOP/php_oop_basic1.php on line 27
Notice: Undefined variable: miles in /home/heidi/Desktop/CodingDojo/OOP/php_oop_basic1.php on line 35
Notice: Undefined variable: price in /home/heidi/Desktop/CodingDojo/OOP/php_oop_basic1.php on line 21
Notice: Undefined variable: max_speed in /home/heidi/Desktop/CodingDojo/OOP/php_oop_basic1.php on line 21
Notice: Undefined variable: miles in /home/heidi/Desktop/CodingDojo/OOP/php_oop_basic1.php on line 21

推荐答案

在PHP（以及javascript，python和许多其他语言）中， $ this 在引用类变量时是显式的：

In PHP (and in javascript, python and many others), $this is explicit when referencing class variables:

// local variable
$price
// class variable
$this->price

还可以看看手册。

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