Drupal 7 (and, obviously Drupal 8) breathe by entity. Entities are everywhere and you can't code a site without them. But here is the problem: all entities have ids, but these ids have different names accross Drupal. Just to give you few examples: Entity id in Node is $node->nid, Entity id in User is $user->uid and so on. Inconsistency as you may say. What if you want to write a code which works accross all entities, how to get entity id for every entity type without writing additional code? I'll show you how.
The answer is Drupal API and entity_extract_ids($entity_type, $entity) function. This is a helper function to extract id, vid, and bundle name from an entity. It accepts two parameters: $entity_type, which can be a text value of 'node', 'user' or whatever type of an entity and $entity which is an entity object itself (for example, $node, $user). Function returns a numerically indexed array (not a hash table) containing these elements: 0: primary id of the entity, 1: revision id of the entity, or NULL if $entity_type is not versioned, 2: bundle name of the entity.
So for example, if we have a code which should work for $node and $user entities, we can get their entity_ids in a very standard way:
$entity_ids = entity_extract_ids('node', $node); $entity_id = $entity_ids;
$entity_ids = entity_extract_ids('user', $user); $entity_id = $entity_ids;
Where this function can be useful? I myself find this approach useful in situations like function hook_field_presave($entity_type, $entity, $field, $instance, $langcode, &$items), where hook provides you with $entity_type and $entity object and where you need to get $entity object id to manipulate the data, but object can be of a variable type (it can be node, user, other entity) because fields should work with any entity type.