array_diff

gistfile1.php

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

foreach($toData as $toUser) {
  array_push($toArray, $toUser['id']);
}

if(isset($datum['message_tags'])){
  foreach($datum['message_tags'] as $tag) {
    array_push($tagArray, $tag[0]['id']);
  }
}

$result = array_diff($toArray, $tagArray);