Fatal error: Call to undefined function get_header()

I’m making a plugin for wordpress there’s a page in this plugin with this code:

init_print_imagebrowser();

function init_print_imagebrowser()

{
        $outz .=get_header();
        $outz .= '<div class="p-imagebrowser-nav">';
        $outz .=$content.'<p>'."hello".'</p>';
        $outz .= '</div>';
        $outz .=get_sidebar();
        $outz .=get_footer();
        return $outz;
}

but it gives me error in the wordpress functions Fatal error: Call to undefined function get_header()
how to fix it???