php code not working properly

Well I have some php code that has a button that updates some text in .txt file. So I installed the plug-in Exec-PHP and I copied my code into the edit view on my page and the button shows up. So far the php plug-in is doing its job. Now for some reason when I click on my button to change the text on a file it redirects me back to my blogs main index.php page. When using this code on a normal .php page it works great. But when using it in wordpress something with how the page is displayed like http://mysite.com/wordpress/?page_id=3 it doesn’t work.

Does anyone have any ideas on either how to create a new separate page or for this code to work properly?

Here is code, hopefully it shows up correctly:

<?php
if($_POST['Submit']){
$open = fopen("test.txt","a+");
$text = "Left\n";
fwrite($open, $text);
fclose($open);
header("Location: {$_SERVER['PHP_SELF']}");
}

echo "<form action=\"".$_SERVER['PHP_SELF']."\" method=\"post\">";
echo "<input name=\"Submit\" type=\"submit\" value=\"Left\" />\n
</form>";
?>