Execute PHP code before any files using .htaccess

[Slyke]

Not sure if this is the right place for this, but I need to be able to run some PHP code before anything loads up on the server.
So far I have a .htaccess file with the following in it:

Options +FollowSymLinks
RewriteEngine On
RewriteBase /
RewriteRule (.*) auth.php?torun=$1 [QSA]

And in auth.php I have this:

<?php
$method="http";
$domain="example.com";


error_reporting(E_ALL);
ini_set('display_errors', 1);


if ($_GET['grantaccess']=="true") {
  $url=$method . '://' . $domain . '/' . $_GET['torun'];
  unset($_GET['torun']);
  unset($_GET['grantaccess']); //Comment out to prevent loop
  if (count($_GET) >= 1) {
    $queryString = "?" . http_build_query($_GET);
  } else {
    $queryString = "";
  }
  //header ( 'Location: ' . $url . $queryString);
  printf ( 'Location: ' . $url . $queryString);
} else {
  echo "Access denied";
}

So far it's working right up to the point where it runs header function. Once that's run there are 1 thing that goes wrong:
It re-executes the .htaccess file again (Which will cause an infinite loop if the "grantaccess" query string is put on). I basically want to somehow disable it re-executing the .htaccess file, or make it only run once per page load/connection.

Edit:
Fixed one of the problems myself, and updated the code.