From php doc
declaration may include an initialization, but this initialization must be a constant value--that is, it must be able to be evaluated at compile time and must not depend on run-time information in order to be evaluated.
You should initialise $model
property in class constructor
public function __construct()
{
// I guess you'll need to call parent constructor as well
parent::__construct();
$this->model = ucfirst($this->database) . '_model';
}