drupal/core/modules/link/link.module

<?php

/**
 * @file
 * Defines simple link field types.
 */

/**
 * Implements hook_help().
 */
function link_help($path, $arg) {
  switch ($path) {
    case 'admin/help#link':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('The Link module allows you to create fields that contain external URLs and optional link text. See the <a href="!field">Field module help</a> and the <a href="!field_ui">Field UI help</a> pages for general information on fields and how to create and manage them. For more information, see the <a href="!link_documentation">online documentation for the Link module</a>.', array('!field' => \Drupal::url('help.page', array('name' => 'field')), '!field_ui' => \Drupal::url('help.page', array('name' => 'field_ui')), '!link_documentation' => 'https://drupal.org/documentation/modules/link')) . '</p>';
      $output .= '<h3>' . t('Uses') . '</h3>';
      $output .= '<dl>';
      $output .= '<dt>' . t('Managing and displaying link fields') . '</dt>';
      $output .= '<dd>' . t('The <em>settings</em> and the <em>display</em> of the link field can be configured separately. See the <a href="!field_ui">Field UI help</a> for more information on how to manage fields and their display.', array('!field_ui' => \Drupal::url('help.page', array('name' => 'field_ui')))) . '</dd>';
      $output .= '<dt>' . t('Adding link text') . '</dt>';
      $output .= '<dd>' . t('In the field settings you can define additional link text to be <em>optional</em> or <em>required</em> in any link field.') . '</dd>';
      $output .= '<dt>' . t('Displaying link text') . '</dt>';
      $output .= '<dd>' . t('If link text has been submitted for a URL, then by default this link text is displayed as a link to the URL. If you want to display both the link text <em>and</em> the URL, choose the appropriate link format from the drop-down menu in the <em>Manage display</em> page. If you only want to display the URL even if link text has been submitted, choose <em>Link</em> as the format, and then change its <em>Format settings</em> to display <em>URL only</em>.') . '</dd>';
      $output .= '<dt>' . t('Adding attributes to links') . '</dt>';
      $output .= '<dd>' . t('You can add attributes to links, by changing the <em>Format settings</em> in the <em>Manage display</em> page. Adding <em>rel="nofollow"</em> notifies search engines that links should not be followed.') . '</dd>';
      $output .= '<dt>' . t('Validating URLs') . '</dt>';
      $output .= '<dd>' . t('All links are validated after a link field is filled in. They can include anchors or query strings. Links need to start with either the scheme name <em>http</em> or <em>https</em>; other scheme names (for example ftp or git) are not supported.') . '</dd>';
      $output .= '</dl>';
    return $output;
  }
}

/**
 * Implements hook_theme().
 */
function link_theme() {
  return array(
    'link_formatter_link_separate' => array(
      'variables' => array('title' => NULL, 'url_title' => NULL, 'href' => NULL, 'options' => array()),
      'template' => 'link-formatter-link-separate',
    ),
  );
}

/**
 * Prepares variables for separated link field templates.
 *
 * This template outputs a separate title and link.
 *
 * Default template: link-formatter-link-separate.html.twig.
 *
 * @param array $variables
 *   An associative array containing:
 *   - title: (optional) A descriptive or alternate title for the link, which
 *     may be different than the actual link text.
 *   - url_title: The anchor text for the link.
 *   - href: The link URL.
 *   - options: (optional) An array of options to pass to l().
 */
function template_preprocess_link_formatter_link_separate(&$variables) {
  if (!empty($variables['title'])) {
    $variables['title'] = check_plain($variables['title']);
  }
  $variables['link'] = l($variables['url_title'], $variables['href'], $variables['options']);
}