在另一个函数中调用Class中的函数

I'd like to understand better how calls to functions work in OOP. I have the following sample:

class SomeClass {
    function __construct(){
        //run function do()
        //run function include()
        //run function run()
    }

    public function do($foo){
        //do some stuff
    }

    public function include(){
        require_once( CONSTANT . 'required.php' );
    }

    public function run(){
        required_func();
    }
}

$load_class = new SomeClass();

in required.php:

function required_func(){

    $customerInfo = "info";
    $customer = $this -> do($customerInfo); //--> This isn't right
    return $customer;
}

What I'm trying to do is to have required_func() run the do() with the $customerInfo. So essentially: How to call a Class public function from another function included in the require_once file? Am I even remotely on track here?

Thanks for your help

$this isn't in scope for function required_func()

class SomeClass {
    function __construct(){
        //run function do()
        //run function include()
        //run function run()
    }

    public function do($foo){
        //do some stuff
    }

    public function include(){
        require_once( CONSTANT . 'required.php' );
    }

    public function run(){
        required_func($this);
    }
}

$load_class = new SomeClass();

and

function required_func($customerObject){

    $customerInfo = "info";
    $customer = $customerObject->do($customerInfo);
    return $customer;
}