Drupal 7 Entities - Create an entity using the entity_metadata_wrapper class.

entity_wrapper_create.php

<?php
  // The entity and bundle we are working with
  $entity_type = "node";
  $bundle = "node";

  // Instantiate the wrapper for the bundle
  $entity = entity_create($entity_type, array('type' => $bundle));
  $wrapper = entity_metadata_wrapper($entity_type, $entity);

  // Check what fields are available to us.
  dsm($wrapper->getPropertyInfo(), 'Available properties for ' . $bundle);
  
  // Update the field values.
  $wrapper->field_a_custom_field_in_our_bundle->set($code);

  // Setting a multi-field for entity reference
  $wrapper->field_entity_reference_field->set($code);
  $wrapper->save();

  return $entity->nid;