代码点火器验证码帮助程序创建验证码时出错

我从我的寄存器控制器调用验证码模型。"insert_captcha（） 模型函数加载验证码帮助程序，更新验证码表，并将创建的验证码返回到注册控制器，将其显示在视图中。

它在 50% 的时间内工作正常，但有时我会收到一大堆 php 错误：

模型摘录：

class captcha_model extends CI_Model {

public function __construct() {
    parent::__construct();
    $this->load->helper('captcha');
}
public function insert_captcha() {
    //captcha creation
    $vals = array(
        'img_path'      => './captcha/',
        'img_url'       =>   base_url() . 'captcha',
        'font_path'     => './assets/fonts/OpenSans-Bold.ttf',
        'img_width'     => '150',
        'img_height'    => 30,
        'expiration'    => 900,
        'word_length'   => 8,
        'font_size'     => 16,
        'img_id'        => 'captcha-pic',
        'pool'          => '0123456789abcdefghijklmnopqrstuvwxyz',
        'colors'        => array(
            'background' => array(255, 255, 255),
            'border' => array(255, 255, 255),
            'text' => array(0, 0, 0),
            'grid' => array(255, 40, 40)
        )
    );
    $captcha = create_captcha($vals);
    //insert captcha into DB
    $captcha_db_params = array(
        'captcha_time'  => $captcha['time'],
        'ip_address'    => $this->input->ip_address(),
        'word'          => $captcha['word']
    );
    $query = $this->db->insert_string('captcha', $captcha_db_params);
    if ( $this->db->query($query) ) { return $captcha;  }           
    else { return false; } 
}
}

控制器的提取

class Register extends CI_Controller {
public $data = array();
public function __construct() {
        parent::__construct();
        $this->load->model('captcha_model');
        if ($this->session->userdata('logged_in')) {  
            $this->data['name'] = $this->session->userdata('name');
            $this->data['id'] = $this->session->userdata('id');
            $this->data['email'] = $this->session->userdata('email');
            $this->data['isAdmin'] = $this->session->userdata('isAdmin');
        }
        if (!empty($this->session->flashdata('error'))) { $this->data['error'] = $this->session->flashdata('error'); }
}
public function index() {
    //captcha creation success
    if ($cap = $this->captcha_model->insert_captcha()) {
        $this->data['title'] = 'Register';
        $this->data['captcha'] = $cap;
        $this->template->load('default', 'register/register', $this->data);
    }
    //not supposed to happen
    else {
        $this->data['error'] = 'Erreur systeme, captcha non genere';
        $this->template->load('default', 'register/register', $this->data);     
    }
}

错误

A PHP Error was encountered
Severity: Notice
Message: **Uninitialized string offset: 36**
Filename: helpers/captcha_helper.php
Line Number: 174
Backtrace:
File: C:'wamp'www'jpweb'models'captcha_model.php Line: 48 Function: create_captcha
File: C:'wamp'www'jpweb'controllers'Register.php Line: 26 Function: insert_captcha
File: C:'wamp'www'index.php Line: 292 Function: require_once A PHP Error was encountered
Severity: Warning
Message: **unpack(): Type C: not enough input, need 1, have 0**
Filename: helpers/captcha_helper.php
Line Number: 174
Backtrace:
File: C:'wamp'www'jpweb'models'captcha_model.php Line: 48 Function: create_captcha
File: C:'wamp'www'jpweb'controllers'Register.php Line: 26 Function: insert_captcha
File: C:'wamp'www'index.php Line: 292 Function: require_once A PHP Error was encountered
Severity: Notice
Message: **String offset cast occurred**
Filename: helpers/captcha_helper.php
Line Number: 206
Backtrace:
File: C:'wamp'www'jpweb'models'captcha_model.php Line: 48 Function: create_captcha
File: C:'wamp'www'jpweb'controllers'Register.php Line: 26 Function: insert_captcha
File: C:'wamp'www'index.php Line: 292 Function: require_once A PHP Error was encountered
Severity: Notice
Message: **Uninitialized string offset: 37**
Filename: helpers/captcha_helper.php
Line Number: 174
Backtrace:
File: C:'wamp'www'jpweb'models'captcha_model.php Line: 48 Function: create_captcha
File: C:'wamp'www'jpweb'controllers'Register.php Line: 26 Function: insert_captcha
File: C:'wamp'www'index.php Line: 292 Function: require_once A PHP Error was encountered
Severity: Warning
Message: **unpack(): Type C: not enough input, need 1, have 0**
Filename: helpers/captcha_helper.php
Line Number: 174
Backtrace:
File: C:'wamp'www'jpweb'models'captcha_model.php Line: 48 Function: create_captcha
File: C:'wamp'www'jpweb'controllers'Register.php Line: 26 Function: insert_captcha
File: C:'wamp'www'index.php Line: 292 Function: require_once A PHP Error was encountered
Severity: Notice
Message: **String offset cast occurred**
Filename: helpers/captcha_helper.php
Line Number: 206
Backtrace:
File: C:'wamp'www'jpweb'models'captcha_model.php Line: 48 Function: create_captcha
File: C:'wamp'www'jpweb'controllers'Register.php Line: 26 Function: insert_captcha
File: C:'wamp'www'index.php Line: 292 Function: require_once