check.php

<?php
include ('Connect.php');

$test = new Connect;
$test->createUser();

?>

Connect.php (unwork)

<?php

class Connect {
    public $connection;

    public function __construct(){
        $this->connect();
    }

    public function connect(){
        $connection = new PDO('mysql:host=orfarile;dbname=orfarile', 'root', '');
    }

    function selectTests(){
        
    }

    public function createUser(){
        $connection->query("INSERT INTO users(user_login, user_password) VALUES ('test1', 'test1')");
    }
}

?>

Connect.php (work)

<?php
class Connect {
    private $connection;

    function __construct(){
        $this->connect();
    }

    function connect(){
        $connection = new PDO('mysql:host=orfarile;dbname=orfarile', 'root', '');
        $connection->query("INSERT INTO users(user_login, user_password) VALUES ('test2', 'test2')");
    }

    function selectTests(){
    }

    function createUser(){    
    }
}
?>

fixed issue#1.
issue#2:
Fatal error: Uncaught Error: Call to a member function query() on null in ~\Connect.php:19 Stack trace: #0 ~\check.php(5): Connect->createUser() #1 {main} thrown in ~\Connect.php on line 19