[PHP] How to insert a file in a class?

[LAMP]

Hi to all.

Let's say there is a class
class Box
{
        var $box_title;
        var $box_content;

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


        function box_title($title)
        {
                $this->title = $title;
        }

        function box_content($content)
        {
                $this->content = $content;
        }

        function make_box()
        {
                $this->box = '<h3>'.$this->box_title.'</h3>'.$this->box_content;
        }


        function get_box()
        {
                return $this->box;
        }
}


$box = new Box();
$box->box_title('PHP Classes');
$box->box_content('Starting with PHP 5, the object model was rewritten  
to allow for better performance and more features. This was a major  
change from PHP 4. PHP 5 has a full object model.')
$box->make_box();
echo $box->get_box();

This works fine.

The problem I have is how to "include" a file as box_content? it could  
be plain text, but it could be a form or some kind of code.
$box->box_include(include(/path/to/file/file.php)) doesn't work, of  
course.

Wrapping up the whole code in a variable doesn't make a sense too:
# file.php
$content = '
        <form method="post" action="$_SERVER['PHP_SELF']">
                Email = <input type=text name=email>
                Pass = <input type=password name=pass>
                <input type=submit value=Submit>
        </form>';

# main.php
$box = new Box();
$box->box_title('PHP Classes');
include(file.php);
$box->box_content($content);
$box->make_box();
echo $box->get_box();


Also, I'm sure I read once it's not correct to print directly from a  
class. First return a value/result to "main" code and then print.  
Correct?

LAMP