get header doesnt work

I use a theme for my website.
I want to load a php file which echos some mysql database value. But I want to keep the theme template. So I added the
<?php get_header(); ?>
my php code
<?php get_sidebar(); ?>
<?php get_footer(); ?>
in the php file, and put that file in the domain root folder.
But when the page loads the php file, a blank page appears. I don’t know what wrong it is.
I also tried copy the theme’s 404 file code into my php file, but it still doesnt work.
I tried to load the 404 file direcity(I copy it under the root folder), it is still a blank page.
The 404.php file of that theme is attached below. Thanks!
<?php get_header(); global $wp_theme_options; ?>
<!–404.php–>

<div id=”container” class=”clearfix”>

<div id=”content”>
<div class=”entry”>
<h1 class=”pagetitle”><?php _e(“Page Not Found”); ?></h1>
<p><?php _e(“We’re sorry, but the page you are looking for isn’t here.”); ?></p>
<p><?php _e(“Try searching for the page you are looking for or using the navigation in the header or sidebar”) ?></p>
</div>
</div><!–end content div–>

<?php get_sidebar(); ?>

<?php get_footer(); ?>