How to introduce another class into a class in php

Sometimes you need to call a method in another class in one class,

and then another class needs to call a method in the current class, what should you do?

You can directly introduce a class object to call a method in another class

The example is as follows (value passing method)

class a {
    function b($obj) {
        $obj->test();
    }
}
  
class b {
    function test() {
        echo 'test';
    }
}
  
$a = new a();
$b->b(new b());

Inheritance method, if the same method is defined in the subclass, it will override the method of the parent class

class b {
    function __construct(){
      
    }
  
    function testb(){
        echo 'test';
    }
}
class a extends b {
    function __construct(){
        parent::testb();
        //or like this
        $this->testb();
    }
    //重复定义 将会覆盖
    function testb(){
        echo 123;
    }
}
  
$a = new a();