运行ajax后我无法从php得到响应

Question

I'm using jQuery and PHP to record comments. The jQuery part works fine, but I can't get PHP ready to record comments. jQuery returns alert 200 after sending, but I think I can't access to 'class' in PHP.

PHP:

class comments extends config{
    public function Action2($id,$comment){
        //record comment
    }
}

$input=json_decode(file_get_contents('php://input'));
$new_comment=new comments();
$new_comment->Action2($input->id,$input->comment);

jQuery:

$.ajax({
    url:'./comment.php',
    type:'POST',
    dataType:'json',
    cache:false,
    contentType:false,
    processData:false,
    data:JSON.stringify({
        'comment':comment,
        'id':id
    }),
});