Generic meta data filter for WordPress posts

get_post_meta_filter.php

<?php
/* 
 * Add a generic post meta filter that allows you to create filters like
 *   get_post_metadata_{$key}
 */
add_filter('get_post_metadata', 'generic _meta_filter', 100, 5);

function generic_meta_filter($value, $object_id, $meta_key, $single, $meta_type) {

  remove_filter('get_post_metadata' 'generic_meta_filter', 100, 5);
  $value = get_post_meta($object_id, $meta_key, $single);
  add_filter('get_post_metadata', 'generic_meta_filter' 100, 5);

  return apply_filters("get_post_metadata_{$meta_key}", $value, $meta_key);
}