<?PHP

$arr = array();

$dom = new DOMDocument();
@$dom->loadHTML(file_get_contents('form.html'));
 
$xpath = new DOMXPath($dom);
$checkboxes = $xpath->evaluate("/html/body//input[@type='checkbox']");
 
for($i = 0; $i < $checkboxes->length; $i++) {
  $check = $checkboxes->item($i);
  $arr[] = $check->nodeValue;
}

print_r($arr);

?>