passing querystrings to a php script from a wp page

I installed exec-php and the Hello World test worked fine. I’m trying to run the php code listed below. My problem is that categories.php?category=Accountant does not work, because there is no categories.php, really. What I want is a way for this to work in WP. How can I get the page saved as categories.php to make the querystring-passing work? Or is there another way?

<?php

dbh=mysql_connect ("localhost", "<USERNAME>", "<PASSWORD>") or die ('I cannot connect to the database because: ' . mysql_error());
mysql_select_db ("intore_cabd");*/

// display individual record
if ($category)
{
   $result = mysql_query("SELECT * FROM <code>businesses</code> WHERE <code>category</code> = '{$category}' ORDER by bizname;", $dbh);
   $myrow = mysql_fetch_array($result);
   echo "<h2>Category: $category</h2>";
   do
   {
      printf("<p><b>%s</b>\n<br />", $myrow["bizname"]);
      printf("%s\n<br />", $myrow["description"]);
      printf("%s\n<br />", $myrow["address"]);
      printf("%s\n<br />", $myrow["country"]);
      printf("%s\n<br />", $myrow["telephone"]);
      if ($myrow["url"])
        printf("<a href=\"$myrow[7]\" target=\"_blank\">%s</a>\n<br />", $myrow["url"]);
      if ($myrow["email"])
        printf("<a href=\"mailto:$myrow[8]\">%s</a>\n</p><br />", $myrow["email"]);
      echo"<hr>";
    }while($myrow = mysql_fetch_array($result));
}
else
{// show categories list
   $result = mysql_query("SELECT * FROM categories ORDER BY category",$dbh);

    if ($myrow = mysql_fetch_array($result))
    {// display list if there are records to display
      do
      {
        printf("<a href=\"%s?category=%s\">%s</a><br>\n", $PHP_SELF, $myrow["category"], $myrow["category"]);
      } while ($myrow = mysql_fetch_array($result));
    }
    else
    {// no records to display
      echo "Sorry, no records were found!";
    }
}

?>