ThinkPHP小白POST无效,该怎么解决

$username = $this->$_POST['username'];
//$name = $this->_post('name'); 换成这行业无效。
$user   =   M('user');
$data =   $user->where('username="'.$username.'"')->field('id,username')->find(); 
if($data) {
	$this->success('成功');
}else{
	$this->error('失败');
}