What is the right way, if possible, to call a class inside another class?

Question

PHP

In class " B " I can't call a function of class " A ". To call it, you need to redefine it $a = new A() in the " B " class. How can I call so as not to override? or maybe you need to somehow correctly describe the class " A "?
index.php

$a = new A();
$b = new B();

A.php

class A {	
  public function print_tst() {
    echo "tst";
  }
}

b.php

class B {	
  function __construct() {
    $a->print_tst();
  }
}

Answer 1

1) static class A
here

class B {	
  function __construct() {
    $a->print_tst();
  }
}

who and how should guess what a is an instance of class A? respectively:
2) When calling, you need to create an instance of class A, otherwise such a call does not make sense.
3) DI - pass the created instance A to the constructor

Answer 2

Create an instance of class "a" global.

Answer 3

__construct($a)