Render a button in a block
I think the classes should be added in the attributes array:

 $button['my_tickets'] =array(
    '#type'       =&gt; 'link',
    '#title'      =&gt; t('My tickets'),
    '#href' =&gt; '',
    '#attributes' =&gt; array('class' =&gt; array('btn', 'btn-default')),
    '#id' =&gt; '',
    '#options' =&gt; array(
      'html' =&gt; true,
    )
  );