Learn the three major characteristics of PHP object-oriented (fully understand abstraction, encapsulation, inheritance, polymorphism)_PHP tutorial

Copy code The code is as follows:

As can be seen from the above, inheritance is a subclass (Subclass) extending the parent class to the public and protected in the parent class (BaseClass) The attributes and methods continue and cannot inherit private attributes and methods
Syntax structure:
class parent class name {}
class subclass name extends parent class name {}
Details:
1 , a subclass can only inherit one parent class (here refers to direct inheritance); if you want to inherit the attributes and methods of multiple classes, you can use multi-level inheritance
Example:

2. When creating a subclass object, the constructor of its parent class will not be automatically called by default
Example:
class A{
public function __construct(){
echo 'A';
}
}
class B extends A{
public function __construct(){
echo 'B ';
}
}
$b=new B();//The constructor in B will be output first. If there is no constructor in B, the
in A will be output. 3. If you need to access the methods of the parent class in a subclass (the modifiers of constructors and member methods are protected or private), you can use parent class::method name or parent::method name to complete [here parent and the previous premise] The self received are all lowercase, and an error is reported in uppercase]
class A{
public function test(){
echo 'a_test';
}
}
class B extends A{
public function __construct(){
//Both methods will work
A::test();
parent::test();
}
}
$b=new B();
5. If the method of a subclass (derived class) is exactly the same as the method of the parent class (public, protected), we call it method coverage or method override. Look at the polymorphism below
3. Polymorphism
Example:

Summary:
1. When a parent class knows that all subclasses have a method, but the parent class is not sure how to write the method, you can let For a subclass to override its method, method overriding (rewriting) must require that the method name and number of parameters of the subclass are exactly the same
2. If the subclass wants to call a method of the parent class (protected/public), You can use parent class name::method name or parent::method name
3. When implementing method rewriting, the access modifiers can be different, but the access rights of the subclass method must be greater than or equal to the access of the parent class method. Permissions (that is, the access permissions of the parent class method cannot be reduced)
For example, if the parent class public function cry(){} and the subclass protected function cry(){} will report an error
But the access permissions of the subclass can be enlarged, such as :
Parent class private function cry(){} subclass protected function cry(){} can execute correctly
Extension:
Method overload (overload)
Basic concept: The function name is the same, but The number of parameters or the type of parameters are different. When the same function is called, different functions can be distinguished
Although overloading is also supported in PHP5, it is still very different from other languages. Multiple functions cannot be defined in PHP. The function of the same name
PHP5 provides powerful "magic" functions. Using these magic functions, we can overload functions.
Here we have to go to __call, when an object calls a method, and the If the method does not exist, the program will automatically call __call
[Officially not recommended]
There are the following magic constants in PHP: __LINE__ __FILE__ __DIR__ __FUNCTION__ __CLASS__ etc.
Example: