Store class instance in public variable into another class php

I’m new to OOP and I’m using twig.
However I’ve two classes ( different files ) and I want to call some functions from for example classA.php into classB.php.

Now I’m creating a new instance inside classB functions each time I want to call functions from classA.

My question now is.. is it possible to create a public variable in classB to create instance then just call that variable inside any function ?

For example:

classB.php

<?php

use AppModelsClassA

class ClassB extends Base
{
public $classA = new ClassA();

public function test
{
return $classA->functionA();
}
}

I’m trying the above, and I got the following error:

Fatal error: Constant expression contains invalid operations

I don’t know if it’s possible or correct, I need your advise.

Thanks in advance.