pain.php

<?php

// HOW TO CREATE A NODE
$node = new stdClass();
$node->title = 'the title';
$node->type = 'node_type';
node_object_prepare($node);
$node->language = LANGUAGE_NONE;
$node->uid = 1;
$node->status = 1;
$node->promote = 0;
$node->comment = 2;
$node = node_submit($node);
node_save($node);

// HOW TO CREATE A USER
$fields = array(
  'name' => 'user_name',
  'mail' => 'email@example.com',
  'pass' => 'password',
  'status' => 1,
  'init' => 'email address',
  'roles' => array(
    DRUPAL_AUTHENTICATED_RID => 'authenticated user',
  ),
);
user_save(null, $fields);

// HOW TO CREATE AN ENTITY THAT USES ENTITY API (for example, commerce_order)
$order = commerce_order_new()
commerce_order_save($order)