Fix dynamic property deprecation warnings

[itismadness]

Fixes deprecation warnings around dynamic properties shown when running this library under PHP 8.2.

Essentially, this is deprecated:

class User {
    public function __construct() {
        $this->name = 'test';
    }
}

where name must be declared as a class property:

class User {
    private $name;
    public function __construct() {
        $this->name = 'test';
    }
}

[serhack]

Thank you very much!