mh-slug-rewrite.php

<?php
/*
Plugin Name: Slug Rewrite
*/

add_action( 'template_redirect', 'mh_slug_rewrite' );
function mh_slug_rewrite() {

// http://website.com/?slug=product_info.php&products_id=32 to http://website.com/products/32
   if ( strpos( $_SERVER['REQUEST_URI'], '?slug=product_info.php&products_id=' ) ) {
      wp_redirect( str_replace( '?slug=product_info.php&products_id=' , '/products' , $_SERVER['REQUEST_URI'] ) );
      exit();
   }

// http://website.com/?slug=register.php to http://website.com/register
   if ( strpos( $_SERVER['REQUEST_URI'], '?slug=register.php' ) ) {
      wp_redirect( str_replace( '?slug=register.php' , '/register' , $_SERVER['REQUEST_URI'] ) );
      exit();
   }

// http://website.com/?slug=index.php&cPath=1 to http://website.com/category/1
   if ( strpos( $_SERVER['REQUEST_URI'], 'http://website.com/category/1' ) ) {
      wp_redirect( str_replace( 'http://website.com/category/1' , '/register' , $_SERVER['REQUEST_URI'] ) );
      exit();
   }
}