Demonstrates a pure function and a wrapper to deal with impurities

capitalize_the_post_title.php

/**
 * Wrapper to deal with global state and pass it to a pure function
 * IMPURE - references global $post
 * @return string
 */
function capitalize_the_post_title() {
  global $post;
  return _capitalize_the_post_title( $post );
  // Or return _capitalize_the_post_title( $GLOBALS['post'] );
}

/**
 * Capitalize a post's title
 * @param WP_Post $post
 * @return string
 */
function _capitalize_the_post_title( $post ) {
  return strtoupper( $post->post_title );
}