One-time callbacks with the WordPress Plugin API

The WordPress Plugin API is a fantastic way for third-party scripts to be able to inject themselves into the WordPress lifecycle; want to change the output of the post content? Simply attach a callback to the the_content filter:

 * Inject a cat emoji at the end of the post content.
 * @param string $content The post content.
 * @return string The filtered post content.
function inject_cat_emoji( $content ) {
  return $content .= ' ?';
add_filter( 'the_content', 'inject_cat_emoji' );

Now, whenever content is run through the the_content filter, our “?” will be appended.