<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class User extends CI_Controller {
    public function login() {
        echo $this->input->post('openid');
    }
}

CREATE TABLE `user` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `username` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `openid` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
  `accessToken` char(32) COLLATE utf8_unicode_ci DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `username` (`username`),
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

database.php

$db['default'] = array(
    'dsn'    => '',
    'hostname' => 'localhost',
    'username' => '',
    'password' => '',
    'database' => '',
    );
数据库类

    public function login() {
        header("Content-type: application/json");
        // 取出参数
        $openid = $this->input->post('openid');
        // 查询数据库
        $query = $this->db->query("select * from user where openid = '" . $openid . "'");
        // 返回行数
        if ($query->num_rows() > 0) {
            // 取出该用户
            $user = $query->first_row();
            // 输出用户的信息
            echo json_encode($user);
            return;
        }
    }
新建代码：

       
 // 注册用户
        $user = array(
                'username' => $openid,
                'openid' => $openid,
                'accessToken' => md5(time().'mysalt')
            );
        $this->db->insert('user', $user);
        echo json_encode($user);
        return;