为什么$ this-＆gt; form_validation-＆gt; run（）总是在codeigniter 3.0.0中使用php7.1在ubuntu 16.04上返回false？

Question

这是代码的一部分，它让我无法登录，不知道问题出在哪里。当我尝试登录时，它没有任何效果。对不起，我的英语不好。

public function login()
{
    if ($this->identity->is_admin())
        redirect('admin/dashboard');
    if ($this->identity->is_contestant())
        redirect('contestant/dashboard');

    $this->form_validation->set_rules('form[username]', $this->lang->line('username'), 'trim|required|max_length[50]');
    $this->form_validation->set_rules('form[password]', $this->lang->line('password'), 'trim|required|max_length[50]');

    if ($this->form_validation->run())
    {
        $credentials = $this->input->post('form');

        if ($this->identity->login($credentials))
        {
            if ($this->identity->is_admin())
                redirect('admin/dashboard');
            else
                redirect('contestant/dashboard');
        }
        else
        {
            $this->session->set_flashdata('error', $this->lang->line('wrong_credentials'));
            redirect('site/login');
        }
    }
    else
    {
        $this->ui['header']['title'] = $this->lang->line('login');
        $this->ui['header']['page'] = 'login';
        $this->load->view('site/header', $this->ui['header']);
        $this->load->view('site/login', $this->ui['content']);
        $this->load->view('footer', $this->ui['footer']);
    }
}

以下是登录网站源代码。

<div class="container">
<div class="row">
    <div class="col-md-12">
        <ul class="breadcrumb">
            <li>
                <i class="glyphicon glyphicon-off"></i> <?php echo $this->lang->line('please_login'); ?>
            </li>
        </ul>
    </div>
</div>

<?php if ($this->session->flashdata('error')) : ?>
    <div class="row">
        <div class="col-md-5">
            <div class="alert alert-danger">
                <?php echo $this->session->flashdata('error'); ?>
            </div>
        </div>
    </div>
<?php endif; ?>

<div class="row">
    <div class="col-md-12">
        <form class="form-horizontal" action="" method="post">
            <div class="form-group<?php echo form_error('form[username]') ? ' has-error' : ''; ?>">
                <label class="col-sm-1 control-label"><?php echo $this->lang->line('username'); ?>:</label>
                <div class="col-sm-4">
                    <input name="form[username]" type="text" maxlength="30" class="form-control" value="<?php echo set_value('form[username]'); ?>" autofocus="on"/>
                    <span class="help-block"><?php echo form_error('form[username]'); ?></span>
                </div>
            </div>
            <div class="form-group<?php echo form_error('form[password]') ? ' has-error' : ''; ?>">
                <label class="col-sm-1 control-label"><?php echo $this->lang->line('password'); ?>:</label>
                <div class="col-sm-4">
                    <input name="form[password]" type="password" maxlength="30" class="form-control"/>
                    <span class="help-block"><?php echo form_error('form[password]'); ?></span>
                </div>
            </div>
            <div class="form-actions col-sm-offset-1">
                <button type="submit" class="btn btn-danger col-sm-3"><i class="glyphicon glyphicon-user"></i> <?php echo $this->lang->line('login'); ?></button>
            </div>
        </form>
    </div>
</div>

是不是CI 3.0.0不支持php7.1所以错误发生了？