PHP表单生成器类在填写表单之前显示错误

I'm using PFBC with validations and when I initially load the form and submit it, it's fine. However, if I then go back to the form and try it again, it displays errors as soon as the form loads (i.e. before I've tried submitting anything).

session_start();

use PFBC\Form;
use PFBC\Element;
use PFBC\Validation;

include("../../PFBC/Form.php");

if(Form::isValid("webform", true)) {
  print "form ok";
  Form::clearValues("webform");
}
else {
  header("redirecturl.php");
}

$form = new Form("webform");

$form->configure(array(
    "prevent" => array("bootstrap")
));

$form->addElement(new Element\Textbox("Name:", "name", array("required" => 1, "size" => 40)));
$form->addElement(new Element\Email("E-mail:", "email", array("required" => 1, "size" => 40)));
$form->addElement(new Element\Textarea("Problem:", "problem", array("required" => 1, "style" => "width: 400px; height: 200px;")));

$form->addElement(new Element\Button("Submit Form", "submit", array("id" => "submit")));

$form->render();