How to define and apply rules for PHP abstract classes

For friends who are working hard to learn PHP (http://www.maiziedu.com/course/php/), the most difficult knowledge to understand and master should be PHP abstract applications. As a novice, I have not yet used object-oriented knowledge to program. However, when developing PHP in the future, it is inevitable to use classes to encapsulate or use interfaces to develop programs in various modular formats.
In PHP, we abstract a class to indicate the general behavior of the class. This class should be a template, which indicates some behaviors that its sub-methods must implement.
Definition of PHP abstract class application:
abstract class ClassName{
}
Key points for PHP abstract class application:
1. Define some methods. Subclasses must fully implement all methods in this abstraction
2. Objects cannot be created from abstract classes, their meaning is to be extended
3. Abstract classes usually have abstract methods, and there are no braces in the methods
PHP abstract class application focus:
1. Abstract methods do not have to implement specific functions and are completed by subclasses
2. When a subclass implements a method of an abstract class, the visibility of the subclass must be greater than or equal to the definition of the abstract method
3. Methods of abstract classes can have parameters or be empty
4. If the abstract method has parameters, then the implementation of the subclass must also have the same number of parameters
PHP abstract class application example:
abstract public function_name(); //Note there are no curly brackets
PHP abstract class rules:
1. As long as a class contains at least one abstract method, it must be declared as an abstract class
2. Abstract methods cannot contain function bodies
3. Inheriting a subclass of an abstract class and implementing an abstract method must have the same or lower access level as the abstract method
4. Inherit a subclass of an abstract class. If it does not implement all abstract methods, then the subclass is also an abstract class
As a demonstration, let's implement a simple abstract class: calculate the area of ​​a rectangle. This rectangle can be extended from the shape class.
abstract class Shape {
abstract protected function get_area();
//Different from the general method, this method does not have curly brackets
//You cannot create an instance of this abstract class: $Shape_Rect= new Shape();
}
class Rectangle extends Shape{
private $width;
private $height;
function __construct($width=0,
$height=0){

​$this->width=$width;
​ $this->height=$height;
}
function get_area(){
echo ($this->width+$this->height)*2;
}
}
$Shape_Rect = new Rectangle(20,30);
$Shape_Rect->get_area();
?>
This is also a simple example, which basically illustrates the usage of abstract classes in PHP. I don’t want to say more about the rest. Personally, I think abstract classes are generally used in large projects, because I think it has too many "rules" to abide by, making it inconvenient to use! Of course this is just my opinion. I would like to say a few more things. PHP abstract class applications are single-inherited, which means that you can only inherit from one class, but not one class that inherits class A and class B. If you want to implement such a function, you have to use an interface. Related knowledge, I will not discuss the knowledge of PHP interface for the time being! In a word: Single inheritance and multiple interfaces!

This topic was moved by Xiaobei on 2016-4-18 12:28