hook wordpress $_GET

hook_get_p.php

<?php 

// Let say you want to do something with affid in http://mysite.com/about/?affid=1234

add_action('init', 'affiliate_redirect');
function affiliate_redirect() {
  $varname = 'aff_id';
  $weeks = '4';
    if( isset($_GET[$varname]) && '' != $_GET[$varname] ) {
    setcookie('wp_affiliate', $_GET[$varname], time()+648000*$weeks, '/');
    $pageURL = 'http';
    if ( isset( $_SERVER["HTTPS"] ) && strtolower( $_SERVER["HTTPS"] ) == "on" ) {
      $pageURL .= "s";
    }
    $pageURL .= "://";
    if ($_SERVER["SERVER_PORT"] != "80") {
      $pageURL .= $_SERVER["SERVER_NAME"].":".$_SERVER["SERVER_PORT"].$_SERVER["REQUEST_URI"];
    } else {
      $pageURL .= $_SERVER["SERVER_NAME"].$_SERVER["REQUEST_URI"];
    }
    $url = preg_replace('/([?&])'.$varname.'=[^&]+(&|$)/','$1',$pageURL);
    $last = $url[strlen($url)-1];
    if ($last == '?') {
      $url = substr_replace($url ,"",-1);
    }
    wp_redirect($url);
    exit;
  }
}