class Document
{
    protected $type;


    function __construct($type)
    {
        $this->type = $type;
    }

    function Show()
    {
        var_dump('Я - документ Типа -' . $this->type );
    }

    function Change()

    {

    }

}

$document = new Document('word');
$document->Show();

class Document
{
    protected $type;
    protected $string


function __construct($type)
{
    $this->type = $type;
    $this->string = 'Я - документ Типа -'.$type;
}

function Show()
{
    echo $this->string;
}

function Change(string $replaceString)
{
   $this->string = str_replace($this->type, $replaceString, $this->string);
}

$document = new Document('word');
$document->Show();
$document->Change('excel');
$document->Show();