gistfile1.php

$toData = $datum['to']['data'];

// rekey to use user id's as index
$rekeyedToData = [];
foreach($toData as $toUser) {
  $key = $toUser['id'];
  // add an extra 'count' key for user ids that show up more than once
  if(array_key_exists($key, $rekeyedToData)) {
    $rekeyedToData[$key]['count']++;
  } else {
    $rekeyedToData[$key] = $toUser;
    $rekeyedToData[$key]['count'] = 1;
  }
}
// iterate through message_tags, remove the tagged user from $rekeyedToData
// if the same id showed up more than once, decrement it
if(isset($datum['message_tags'])) {
  foreach($datum['message_tags'] as $tag) {
    // each tag object in FB's JSON response is actually wrapped in an array.
    // if FB decides to change this structure in the future, then the following line will break.
    $key = $tag[0]['id'];
    if($rekeyedToData[$key]['count'] > 1) {
      $rekeyedToData[$key]['count']--;
    } else {
      $rekeyedToData[$key] = null;
    }
  }
}

// do the same with with_tags, remove the tagged user from $rekeyedToData

$rekeyedToData = array_filter($rekeyedToData);

// if $rekeyedToData is empty, then this post was a self post, e.g the poster posted on their own wall and tagged a bunch of people
// otherwise the post has a definite recipient
if(!empty($rekeyedToData)) {
  // should never be more than one element in the array
  if(count($rekeyedToData) > 1){
    Logger::log("More than one element in recipient array.");
  }
  $recipient = end($rekeyedToData); // could have used reset, instead of end to get the first element. Assumption is there would only by one element in the array
}