gistfile1.txt

// if field_id_4 is your playa field it works like this

$this->EE->load->library('api');
$this->EE->api->instantiate('channel_entries');
$this->EE->api->instantiate('channel_fields');

$data = array(
  'title' => $entry->title,
  'field_id_4' => '',
  'field_ft_4' => 'none',
  'entry_date' => time(),
  'edit_date' => time()
);

$data['field_id_4']['selections'] = array($site->entry_id);

$this->EE->api_channel_fields->setup_entry_settings(2, $data);
$this->EE->api_channel_entries->submit_new_entry(2, $data);