1. Abstract class in PHP

PHP 5 supports abstract classes and abstract methods. Classes defined as abstract cannot be instantiated. Any class must be declared abstract if at least one method in it is declared abstract. A method defined as abstract only declares its calling method (parameters) and cannot define its specific function implementation. A class can be declared abstract by using the abstract modifier in its declaration.

It can be understood that an abstract class serves as a base class and leaves specific details to successors. By abstracting concepts, you can create scalable architectures in your development projects.

1.1, Abstract method

Use the abstract keyword to define abstract methods. Abstract methods only retain the method prototype (the signature after the method body is removed from the method definition), which includes access levels, function keywords, function names and parameters. It does not contain ({}) or any code inside brackets. For example, the following code is an abstract method definition:

When inheriting an abstract class, the subclass must define all abstract methods in the parent class; in addition, the access control of these methods must be the same (or more relaxed) as in the parent class. In addition, the method calling methods must match, that is, the type and number of required parameters must be consistent.

1.2. About abstract classes

A class must be declared as an abstract class as long as it contains at least one abstract method.
Methods declared as abstract must contain the same or lower access level when implemented.
Instances of abstract classes cannot be created using the new keyword.
Methods declared as abstract cannot contain function bodies.
If the extended class is also declared as an abstract class, you do not need to implement all abstract methods when extending the abstract class. (If a class inherits from an abstract class, it must also be declared abstract when it does not implement all abstract methods declared in the base class.)
1.3. Use abstract classes

使用接口（interface），可以指定某個(gè)類必須實(shí)現(xiàn)哪些方法，但不需要定義這些方法的具體內(nèi)容。

接口是通過 interface 關(guān)鍵字來定義的，就像定義一個(gè)標(biāo)準(zhǔn)的類一樣，但其中定義所有的方法都是空的。

接口中定義的所有方法都必須是公有，這是接口的特性。

接口是一種類似于類的結(jié)構(gòu)，可用于聲明實(shí)現(xiàn)類所必須聲明的方法。例如，接口通常用來聲明API,而不用定義如何實(shí)現(xiàn)這個(gè)API。

大多數(shù)開發(fā)人員選擇在接口名稱前加上大寫字母I作為前綴，以便在代碼和生成的文檔中將其與類區(qū)別開來。

2.1接口實(shí)現(xiàn)（implements）

要實(shí)現(xiàn)一個(gè)接口，使用 implements 操作符（繼承抽象類需要使用 extends 關(guān)鍵字不同），類中必須實(shí)現(xiàn)接口中定義的所有方法，否則會(huì)報(bào)一個(gè)致命錯(cuò)誤。類可以實(shí)現(xiàn)多個(gè)接口，用逗號(hào)來分隔多個(gè)接口的名稱。

實(shí)現(xiàn)多個(gè)接口時(shí)，接口中的方法不能有重名。
接口也可以繼承，通過使用 extends 操作符。
類要實(shí)現(xiàn)接口，必須使用和接口中所定義的方法完全一致的方式。否則會(huì)導(dǎo)致致命錯(cuò)誤。
接口中也可以定義常量。接口常量和類常量的使用完全相同，但是不能被子類或子接口所覆蓋。
2.2使用接口的案例

3、類型運(yùn)算符 instanceof

instanceof 運(yùn)算符是 PHP5 中的一個(gè)比較操作符。他接受左右兩邊的參數(shù)，并返回一個(gè)boolean值。

確定一個(gè) PHP 變量是否屬于某個(gè)一類 CLASS 的實(shí)例
檢查對(duì)象是不是從某個(gè)類型繼承
檢查對(duì)象是否屬于某個(gè)類的實(shí)例
確定一個(gè)變量是不是實(shí)現(xiàn)了某個(gè)接口的對(duì)象的實(shí)例

4.Contract Programming

Design by Contract or Design by Contract (DbC) is a method of designing computer software. This method requires software designers to define formal, precise and verifiable interfaces for software components. In this way, a priori conditions, a posteriori conditions and invariants are added to traditional abstract data types. The "contract" or "contract" used in the name of this method is a metaphor because it is somewhat similar to the situation of a business contract.

A programming practice of implementing a declared interface before writing a class. This method is very useful in ensuring the encapsulation of classes. Using contract programming techniques, we can define the functionality of a view before creating an application, much like an architect draws a blueprint before building a building.

5.Summary

Abstract classes are classes declared using the abstract keyword. By marking a class as abstract, we can defer implementation of the declared methods. To declare a method as abstract, simply remove the method entity containing all curly braces and end the line of code where the method is declared with a semicolon.

Abstract classes cannot be instantiated directly, they must be inherited.

If a class inherits from an abstract class, it must also be declared abstract when it does not implement all abstract methods declared in the base class.

In an interface, we can declare a method prototype without a method body, which is very similar to an abstract class. The difference between them is that interfaces cannot declare any methods with method bodies; and the syntax they use is also different. In order to force uncovering rules on a class, we need to use the implements keyword instead of the extends keyword.

In some cases we need to determine whether a class is a type of a specific class, or whether it implements a specific interface. instanceof is suitable for this task. instanceof checks three things: whether the instance is of a specific type, whether the instance inherits from a specific type, and whether the instance or any of its ancestor classes implement a class-specific interface.

Some languages ??have the ability to inherit from multiple classes, this is called multiple inheritance. PHP does not support multiple inheritance. The idea is that it provides the function of declaring multiple interfaces for a class.

Interfaces are useful for declaring rules that a class must follow. Contractual programming technology uses this feature to enhance encapsulation and optimize workflow.