Detailed explanation of inheritance usage examples of PHP classes

In PHP, I often write a class, the class writes a shared method, and then let the subclass inherit to get the corresponding function. Suppose there is roughly such a parent class:

<?php
class Father{

    public function construct(){
        echo &#39;我是父类的构造方法!&#39;;
    }

    protected function say($str = &#39;Hello World!&#39;){
        echo &#39;说了一句话：&#39; . $str;
    }
}
?>

Then, make a subclass to inherit it:

<?php

class chirld extends Father{

    public construct(){
        echo &#39;我是子类的构造方法~&#39;;
    }
}

At this time, if you instantiate the subclass, the result will be I am the constructor of the subclass~!
Of course, because inherits the relationship, the subclass can call the say() method of the parent class.

If you want to implement the construction method of the parent class, but you also want to implement the construction method of the subclass, you can do it like this:

<?php

class chirld extends Father{

    public construct(){
        parent::construct();
        echo &#39;我是子类的构造方法~&#39;;
    }
}

?>

At this time, I will be output It is the constructor of the parent class! and I are the constructor of the subclass~.

Suppose you define such a method in the subclass:

protected function say($str = ''){
   echo '我是子类的say';
}

Then, the sys() method you inherited from the parent class will be overridden. Write, so the output result after the call is: I am the say of the subclass.

What happens if you define it this way?

public function say($str = ''){
     echo '我是子类的方法哦~';
}

This way of writing is still acceptable. The difference between PHP and other strongly typed languages ​​is that the PHP overriding method allows you to "publicize upwards" the overridden method, but does not allow you to "privatize downwards". For example, here you define it as private will definitely report an error, but languages ​​like C++ are exactly the opposite.

PHP’s thinking about this situation is that your father gave you a protected level inheritance. At this time, it is a protected level in your hands. Your father allows you to share it with others, that ispublic, but you are not allowed to hide it yourself, that is, private, and you definitely can’t keep it all to yourself.

You will definitely ask at this time, what will happen if the parent class's say() is set to private?
The result will not be the same, that is, the parent class has been privatized and the subclass cannot inherit it at all, so you can decide the say() method in the subclass as you like.

There is another problem, that is, the parent class has already defined sys() with optional parameters. So, what will happen if you define it in the subclass without parameters like the following?

public function say(){
    echo '哈哈。我没有参数了耶~';
}

The result can still run normally, but there will be an E_STRICT level prompt. The reason is that the PHP standard is that the number of parameters must be aligned with the parent class. Of course, the error level can be set in php.ini.

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