按照老师的代码来的可是一直报这个错误：500 (Internal Server Error)

//登录（html）
function dologin(){
			var username = $.trim($('#username').val());
			var password = $.trim($('#password').val());
			var verifycode = $.trim($('#verifycode').val());
			if(username == ''){
				layer.alert('请输入用户名',{icon:2});
				return;
			}
			if(password == ''){
				layer.alert('请输入密码',{icon:2});
				return;
			}
			if(verifycode==''){
				layer.alert('请输入验证码',{icon:2});
				return;
			}
			$.post('/admins.php/admins/Account/dologin',{'username':username,'password':password,'verifycode':verifycode},function(res){
				if(res.code>0){
					reloadImg();
					layer.alert(res.msg,{icon:2});
				}else{
					layer.msg(res.msg);
					setTimeout(function(){window.location.href = '/admins.php/admins/Home/index'},1000);
				}
			},'json')
			
//控制器(controller)
public function dologin(){
		$username = trim(input('post.username'));
		$pwd = trim(input('post.password'));
		$verifycode = trim(input('post.verifycode'));

		if($username == ''){
			exit($res = json_encode(array('code'=>1,'msg'=>'用户名不能为空')));
		}
		if($pwd == ''){
			exit($res = json_encode(array('code'=>1,'msg'=>'密码不能为空')));
		}
		if($verifycode == ''){
			exit($res = json_encode(array('code'=>1,'msg'=>'请输入验证码')));
		}
		// 验证验证码
		if(!captcha_check($verifycode)){
			exit($res = json_encode(array('code'=>1,'msg'=>'验证码错误')));
		}
		// 验证用户
		$this->db = new Sysdb;
		$admin = $this->db->table('admins')->where(array('username'=>$username))->item();
		if(!$admin){
			exit($res = json_encode(array('code'=>1,'msg'=>'用户不存在')));
		}
		if(md5($admin['username'].$pwd) != $admin['password']){
			exit($res = json_encode(array('code'=>1,'msg'=>'密码错误')));
		}
		if($admin['status'] == 1){
			exit($res = json_encode(array('code'=>1,'msg'=>'用户已被禁用')));
		}
		// 设置用户session
		session('admin',$admin);
		exit($res = json_encode(array('code'=>0,'msg'=>'登录成功')));
	}