The warning is telling you that there is a property you are trying to set which isn't declared in the class or any of its parents.

When you run this:

class database {

    public $username = "root";
    public $password = "pasword";
    public $port = 3306;

    public function __construct($params = array())
    {
        foreach ($params as value)
        {
            $this->{$key} = $value;
        }
    }
}

$db = new database(array(
    'database' => 'db_name',
    'server' => 'database.internal',
));

It is roughly equivalent to this:

class database {

    public $username = "root";
    public $password = "pasword";
    public $port = 3306;
}

$db = new database;
$db->database = 'db_name';
$db->server = 'database.internal';

The warning is that there is no line in the class definition saying that $db->database or $db->server exist.

For now, they will be dynamically created as untyped public properties, but in future, you will need to declare them explicitly:

class database {
    public $database;
    public $server;
    public $username = "root";
    public $password = "pasword";
    public $port = 3306;

    public function __construct($params = array())
    {
        foreach ($params as value)
        {
            $this->{$key} = $value;
        }
    }
}

$db = new database(array(
    'database' => 'db_name',
    'server' => 'database.internal',
));

In some rare situations, you actually want to say "the properties of this class are whatever I decide to add at run-time"; in that case, you can use the #[AllowDynamicProperties] attribute, like this:

#[\AllowDynamicProperties]
class objectWithWhateverPropertiesIWant {
    public function __construct($params = array())
    {
        foreach ($params as value)
        {
            $this->{$key} = $value;
        }
    }
}

Those notices have already been fixed and the fix will be included in the upcoming 5.69 release.

Define the helper class like this

<?php
namespace ....

use Vendor\Modulename\Helper\Data;


class MyClass
{
    protected $helper;
    
    
    public function __construct(
        Data $helper
    ) {
        $this->helper = $helper;
        
    }
    
}