致命错误：调用未定义的函数form_open（）第104行

I know most of you guys will realize this is a dumb question but I don't have any idea what's wrong with my code. I'm trying to create a contact form that will be sent into the database. But when I try to go to the contact_form_view.php it says Fatal error: Call to undefined function form_open() on Line 104

Here's the form code

<?php $attributes = array("name" => "contactform");
        echo form_open("contactform/index", $attributes);?>
        <div class="form-group">
            <label for="name">Name</label>
            <input class="form-control" name="name" placeholder="Your Full Name" type="text" value="<?php echo set_value('name'); ?>" />
            <span class="text-danger"><?php echo form_error('name'); ?></span>
        </div>

        <div class="form-group">
            <label for="email">Email ID</label>
            <input class="form-control" name="email" placeholder="Email-ID" type="text" value="<?php echo set_value('email'); ?>" />
            <span class="text-danger"><?php echo form_error('email'); ?></span>
        </div>

        <div class="form-group">
            <label for="subject">Subject</label>
            <input class="form-control" name="subject" placeholder="Subject" type="text" value="<?php echo set_value('subject'); ?>" />
            <span class="text-danger"><?php echo form_error('subject'); ?></span>
        </div>

        <div class="form-group">
            <label for="message">Message</label>
            <textarea class="form-control" name="message" rows="4" placeholder="Message"><?php echo set_value('message'); ?></textarea>
            <span class="text-danger"><?php echo form_error('message'); ?></span>
        </div>

        <div class="form-group">
            <button name="submit" type="submit" class="btn btn-success">Submit</button>
        </div>
        <?php echo form_close(); ?>
        <?php echo $this->session->flashdata('msg'); ?>

Here's the php code

<?php
class contactform extends CI_Controller
{
   public function __construct()
   {
        parent::__construct();
       $this->load->helper(array('form','url'));
       $this->load->library(array('session', 'form_validation'));
       $this->load->database();
   }

   function index()
   {
        //set validation rules
        $this->form_validation->set_rules('name',   'Name',                     'trim|required|xss_clean|callback_alpha_space_only');
        $this->form_validation->set_rules('email', 'Emaid ID', 'trim|required|valid_email');
        $this->form_validation->set_rules('subject', 'Subject', 'trim|required|xss_clean');
        $this->form_validation->set_rules('message', 'Message', 'trim|required|xss_clean');

        //run validation on post data
        if ($this->form_validation->run() == FALSE)
        {   //validation fails
            $this->load->view('contact_form_view');
        }
        else
        {
            //insert the contact form data into database
            $data = array(
                'name' => $this->input->post('name'),
                'email' => $this->input->post('email'),
                'subject' => $this->input->post('subject'),
                'message' => $this->input->post('message')
            );

            if ($this->db->insert('contacts', $data))
            {
                // success
                $this->session->set_flashdata('msg','<div class="alert alert-success text-center">We received your message! Will get back to you shortly!!!</div>');
                redirect('contactform/index');
            }
            else
            {
                // error
                $this->session->set_flashdata('msg','<div class="alert alert-danger text-center">Oops! Some Error.  Please try again later!!!</div>');
                redirect('contactform/index');
            }
        }
    }

    //custom callback to accept only alphabets and space input
    function alpha_space_only($str)
    {
        if (!preg_match("/^[a-zA-Z ]+$/",$str))
        {
            $this->form_validation->set_message('alpha_space_only', 'The %s field must contain only alphabets and space');
            return FALSE;
        }
        else
        {
            return TRUE;
        }
    }
}
?>

if you please try to load form helper in your autoload.php like this

$autoload['helper'] = array('url', 'form');

i think you load the helper in the controller which have the action of your form not the controller that call the form.

so just add the helper to autoload.php and it will work