<?php
/*
Plugin Name: Simple Replacer
Plugin URI: link to your page to download the plugin or usage guide
Description: Replaces {donationcode} with paypal donation codes
Author: Zen
Version: 1.0
Author URI: http://zenverse.net/
*/

function the_replacer($content)
{
    $post = str_replace( '{donationcode}' , 'YOUR_DONATION_HTML_CODES' , $content);
    return $content;
}

add_filter('the_content', 'the_replacer');

?>