Question

The global $post variable is not set here. How do I set it?

function fb_comment_count($link = 'link') {
  global $post;
  $url = 'https://graph.facebook.com/';
  $posturl = get_permalink($post->ID);
  $url .= $posturl;

  $filecontent = wp_remote_retrieve_body(wp_remote_get($url, array('sslverify'=>false)));
  $json = json_decode($filecontent);
  $count = $json->comments;
  if ($count == 0 || !isset($count)) {
    $count = 0;
  }

  $comments = $count;
  if ($count == 1) {
    $comments .= '';
  }
  elseif ($count == 0) {
    $comments = '0';
  }
  elseif ($count > 1) {
    $comments .= '';
  }
  if ($link == 'nolink') {
    return $comments;
  }
  else {
    return '<a href="'.$posturl.'#comments" title="Comments for '.$post->post_title.'">'.$comments.'</a>';
  }
}

Answer 1

You “set” it the same way you “set” a regular (non-global) variable, with the assignment operator – an equals character (=).

http://php.net/manual/en/language.operators.assignment.php

You can also use a $GLOBALS[] array. Please see the documentation for what the global keyword actually does:

http://php.net/manual/en/language.variables.scope.php