I got confused with this code

```
<?php

$input = 'O:1:"B":1:{s:6:"secret";s:6:"Got_it";}';
$o = unserialize($input);
//echo $o;  
print_r($o->secret);

?>

```

I have tried to get the $o->secret but it happened this error: 

> 
> **Notice:** main(): The script tried to execute a method or access a
>  property of an incomplete object. Please ensure that the class
>  definition "B" of the object you are trying to operate on was loaded
>  *before* unserialize() gets called or provide a \_\_autoload() function to load the class definition in /home/mikun/public\_html/tested.php on
>  **line 6**
> 
> 
> 

Please help me to sovle it!