Silverstripe 3.1 nice internal / external link field

gistfile1.php

<?php		
		// nice internal / external link field
		$fields->replaceField('LinkExteralOrInternal', (SelectionGroup::create(
					"LinkExteralOrInternal",
					array(
						SelectionGroup_Item::create(
							"external",
							TextField::create(
								"ExternalLink", 
								"Externe link"
							),
							'Externe link'
						),
						SelectionGroup_Item::create(
							'internal',
							TreeDropdownField::create(
								"InternalLinkID", 
								"",
								'SiteTree',
								'ID',
								'TreeTitle'
							),
							'Interne link'
						)
					)
				)
			)
		);