Fatal error: Call to undefined function get_header()

I’m writing my first widget, and I’m having an issue. I’ve written up some html to put up a textbox and select box in my sidebar with a submit button. Off that submit button, I’m trying to activate a new php routine I’m writing. But first, just to test my code, I’m just trying to activate single.php (part of the theme I’m using, “pixeled”). The call I’m using is:
echo “<form method=\”get\” id=\”logsearchform\” action=\”/wp-content/themes/pixeled/single.php\”>”;

I get this error:
Fatal error: Call to undefined function get_header() in /home1/milehig4/public_html/wp-content/themes/pixeled/single.php on line 1

I know that single.php works because I can navigate to any blog page on my site (www.milehighcrossfit.com). So for some reason it can’t see the functions the way I’m calling the routine off my button.

Any thoughts here? Any help would be much appreciated

Steve