title: How to put a html tag (span) inside a button in twig file symfony 2.8?
body: <p>I have been trying to customize a form individual field, following the symfony documentation:
  <a href="http://symfony.com/doc/master/form/form_customization.html#how-to-customize-an-individual-field" rel="nofollow noreferrer">http://symfony.com/doc/master/form/form_customization.html#how-to-customize-an-individual-field</a></p>
  
  <p>But I have not had success in my attempt.
  This is my twig:</p>
  
  <pre><code>{% block _Formulario_personalizado_widget %}
                  &lt;span class="icon-icon-buscar-libros fs1"&gt;&lt;/span&gt;
              {% endblock %}
  
              {{ form_widget(Formulario.Buscar, {'id':"BuscarPorCriterio", 'attr':{'class':'btn btn-primary active'}}) }}
  </code></pre>
  
  <p>This is my form in the cotroller:</p>
  
  <pre><code> -&gt;add('Buscar',ButtonType::class, array('block_name'=&gt; 'personalizado'))
  </code></pre>
  
  <p>And this controller returns a twig with next form name:</p>
  
  <pre><code>array("Formulario"=&gt;$form-&gt;createView())
  </code></pre>
  
  <p>However, the output is like this: </p>
  
  <pre><code>&lt;span class="icon-icon-buscar-libros fs1"&gt;&lt;/span&gt;
  
              &lt;button type="button" id="BuscarPorCriterio" name="form[Buscar]" class="btn btn-primary active"&gt;Buscar&lt;/button&gt;
  </code></pre>
  
  <p>And I actually need this output (with <strong>SPAN</strong> INSIDE <strong>Button</strong>):</p>
  
  <pre><code>&lt;button type="button" id="BuscarPorCriterio" name="form[Buscar]" class="btn btn-primary active"&gt;&lt;span class="icon-icon-buscar-libros fs1"&gt;&lt;/span&gt; Buscar&lt;/button&gt;
  </code></pre>
  
  <p>Thank you in advance for your help!</p>