Function for Post Format icons

post_format_icon.php

<?php
/**
 * Output Post Format icons
 * 
 * Outputs an icon for each post format. Set up to use Fontawesome,
 * but can be used with anything, or you can just use CSS.
 *
 * @return string
 */
function post_format_icon() {

  global $post;

  // get current post ID
  $id     = $post->ID;

  // get post format
  $format = get_post_format( $id );

  // array of icons as $format => $icon key/value pairs
  $icons = [
    'standard' => 'fa-pencil',
    'aside'    => 'fa-sticky-note',
    'chat'     => 'fa-comments',
    'gallery'  => 'fa-picture-o',
    'link'     => 'fa-external-link',
    'image'    => 'fa-camera',
    'quote'    => 'fa-quote-left',
    'status'   => 'fa-commenting',
    'video'    => 'fa-video-camera'
  ];

  // format will return false if no format is set. So, evaluate against this..
  // if format = false then $icon = standard, else icon = selected format
  $format == ( false ) ? $icon = $icons['standard'] : $icon = $icons[ $format ];
  
  // html to be output
  $output = '<span class="fa-stack"><i class="fa fa-circle fa-stack-2x"></i><i class="fa fa-stack-1x fa-inverse ' . $icon . '"></i></span>';

  // return the output
  return $output;
}

It’s not working