Drupal - How do I create a link?

\Drupal::l is deprecated. Maybe this case will be useful for somebody

  use Drupal\Core\Url;
  use Drupal\Core\Link;
  $url = Url::fromRoute('entity.node.edit_form', array('node' => NID));
  $project_link = Link::fromTextAndUrl(t('Open Project'), $url);
  $project_link = $project_link->toRenderable();
  // If you need some attributes.
  $project_link['#attributes'] = array('class' => array('button', 'button-action', 'button--primary', 'button--small'));
  print render($project_link);

One, this is not 100% complete, see this issue . With that said, let me quote some code from the change notice:

Drupal 7:

// Internal path.
$internal_link = l(t('Book admin'), 'admin/structure/book');

// External Uri.
$external_link = l(t('External link'), 'http://www.example.com/', array('external' => TRUE));

Drupal 8:

// Internal path (defined by a route in Drupal 8).
use Drupal\Core\Url;
$url = Url::fromRoute('book.admin');
$internal_link = \Drupal::l(t('Book admin'), $url);

// External Uri.
use Drupal\Core\Url;
$url = Url::fromUri('http://www.example.com/');
$external_link = \Drupal::l(t('External link'), $url);

Edit: route names are in the moduledirectory/modulename.routing.yml files and (by default) in the {router} table.

Here are some examples of creating links in Drupal 8. Note that $this->t('some text') is available within blocks that extend BlockBase. If you copy these to another class that doesn't have it or use these in a .module file, you may need to change this to t() 1.

Basic Link to a node:

$node = Node::load($nid);
$build['node_link'] = $node->toLink()->toRenderable();

This creates a render array like this:

$link = [
  '#type' => 'link',
  '#url' => $url_object,
  '#title' => 'Title of Node',
];

You can create the render array without loading the node this way:

$url_object = Url::fromRoute('entity.node.canonical', ['node' => $nid]);
$link = [
  '#type' => 'link',
  '#url' => $url_object,
  '#title' => $this->t('Read More'),
];

Or using the core Link class:

$url = Url::fromRoute('entity.node.canonical', ['node' => $nid]);
$link = Link::fromTextAndUrl($this->t('Read more'), $url);
$build['read_more'] = $link->toRenderable();

If you want to use markup in the text of your link, you can't just put a string in. You need to use a render array element:

$url = Url::fromRoute('entity.node.canonical', ['node' => $nid]);
$link_text =  [
  '#type' => 'html_tag',
  '#tag' => 'span',
  '#value' => $this->t('Load More'),
];
$link = Link::fromTextAndUrl($link_text, $url);

To create an absolute link, you add this option to the URL, not the link:

$url = Url::fromRoute('entity.node.canonical', ['node' => $nid], ['absolute' => TRUE]);
$link = Link::fromTextAndUrl($this->t('Read more'), $url);
$build['read_more'] = $link->toRenderable();

To add a class to your link, you also need to add this to the URL, not the link:

$options = [
  'attributes' => [
    'class' => [
      'read-more-link',
    ],
  ],
];
$url = Url::fromRoute('entity.node.canonical', ['node' => $nid], $options);
$link = Link::fromTextAndUrl($this->t('Read more'), $url);
$build['read_more'] = $link->toRenderable();

To add a query string to your link, you also need to this to the URL, not the link:

$options = [
  'query' => [
    'car' => 'BMW',
    'model' => 'mini-cooper',
  ],
  'attributes' => [
    'class' => [
      'read-more-link',
    ],
  ],
];
$url = Url::fromRoute('entity.node.canonical', ['node' => $nid], $options);
$link = Link::fromTextAndUrl($this->t('Read more'), $url);
$build['read_more'] = $link->toRenderable();

To set the link to open in a new window with target = _blank:

$options = [
  'attributes' => [
    'target' => '_blank'
  ],
];
$url = Url::fromRoute('entity.media.edit_form', ['media' => $entity->id()], $options);
$link = Link::fromTextAndUrl(t('Edit'), $url);
$form['entity']['edit_link'] = $link->toRenderable();

Here's a link to a taxonomy term page:

$url = Url::fromRoute('entity.taxonomy_term.canonical', ['taxonomy_term' => $tid]);
$link = Link::fromTextAndUrl($this->t('Read more'), $url);
$build['read_more'] = $link->toRenderable();

Here's a link to node edit page:

$url = Url::fromRoute('entity.node.edit_form', ['node' => $nid]);
$link = Link::fromTextAndUrl($this->t('Edit'), $url);
$build['read_more'] = $link->toRenderable();

To create an external link:

$url = Url::fromUri('http://www.example.com/');
$link = Link::fromTextAndUrl($this->t('Vist this example site'), $url);
$build['external_link'] = $link->toRenderable();

Link to homepage:

$url = Url::fromRoute('<front>');
$link = Link::fromTextAndUrl($this->t('Home'), $url);
$build['homepage_link'] = $link->toRenderable();

Note that on any of these url objects, you can get the url as a string by calling:

$url->toString();

For example:

$url_string = Url::fromRoute('<front>')->toString();