How to check a checkbox by default in forms api
See below example...

$options = array();
$options["1"] = "One";
$options["2"] = "Two";
$options["3"] = "Three";
$options["4"] = "Four";


$form['ios'] = array(
  '#title' =&gt; t(''),
  '#type' =&gt; 'checkboxes',
  '#description' =&gt; t(''),
  '#options' =&gt; $options,
  '#default_value' =&gt; array("1", "2", "3")
);