What is "auto casting" and how to use it?

Question

I have the class y:

class y {
        private $group = [];
        public function __construct(){
            $this->group['Start'] = array(
                'GET' => array(
                    'Home' => array(
                        'Method' => 'ControllerHome@index',
                        'Logged' => true,
                        'Parameters' => false
                    )
                )
            );   
        }

        public function GET($g, $function, $logged, $parameters){
            $explode = explode('/', $g);
            if(array_key_exists($explode[0], $this->group)){
                if(!array_key_exists($explode[1],  $this->group[$explode[0]]['GET'])){
                    $this->group[$explode[0]]['GET'][$explode[1]] = array(
                        'Method' => $function,
                        'Logged' => $logged,
                        'Parameters' => $parameters
                    );
                    //var_dump($this->group);
                    $t = new z('a');
                    return $t;
                }
            } else {
                $this->group[$explode[0]]['GET'] = [];
                $this->GET($g, $function, $logged, $parameters);
            }
        }

        public function POST($g, $function, $logged, $parameters){
            $explode = explode('/', $g);
            if(array_key_exists($explode[0], $this->group)){
                if(!array_key_exists($explode[1],  $this->group[$explode[0]]['GET'])){
                    $this->group[$g]['GET'][$explode[1]] = array(
                        'Method' => $function,
                        'Logged' => $logged,
                        'Parameters' => $parameters
                    );
                    var_dump($this->group);
                }
            } else {
                $this->group[$explode[0]] = [];
                $this->x($g, $function, $logged, $parameters);
            }
        }
    }

Just below I have the class z:

class z {
        public function __construct($teste){

        }

        public function r($mensagem){
            return $mensagem;
        }
    }

What I want is to call the function of the other class - in case r().

$a = new y();
$a->GET('k/home', 'ControllerHome@index', true, false)->r('teste');

Is there a specific name for this action? How can I use it? Because I’m getting the error below?

Error: Call to a member function r() on null in C:\laragon\www\nota_veio_novo\index.php on line 70