title: How to create multiple checkboxes grouped by fieldsets in Cakephp 3
body: <p>I'm having a problem following this documentation :</p>
  
  <p><a href="http://book.cakephp.org/3.0/en/views/helpers/form.html#creating-select-pickers" rel="nofollow">Cakephp3 Cookbook - Form - Creating Select Pickers</a>
  I tried the 'multiple checkboxes' part : </p>
  
  <pre><code>$options = [
     'Group 1' =&gt; [
        'Value 1' =&gt; 'Label 1',
        'Value 2' =&gt; 'Label 2'
     ],
     'Group 2' =&gt; [
        'Value 3' =&gt; 'Label 3'
     ]
  ];
  echo $this-&gt;Form-&gt;select('field', $options, ['multiple' =&gt; 'checkbox']);
  </code></pre>
  
  <p>but the output was an error like this : </p>
  
  <pre><code>Notice (8): Array to string conversion [CORE/src/View/StringTemplate.php, line 238]
  </code></pre>
  
  <p>it's like telling me that the value of the array should be a string instead of an Array, but is there anyway to make this work ? Please can anyone help me solve this problem ?</p>