Class 'vB_Api_Extensions' not found

[hienntp]

Hello,
I have another one and I need your help.
I created a mode for my forum (vb5). I used ajax to transfer data from outside to core/packages/helloworld/api/page.php:

PHP Code:

<?php   
class helloworld_Api_Page extends vB_Api_Extensions   
{   
public $product = 'helloworld';   
public $version = '1.0.0';   
public $developer = 'Bkav';   
public $title = 'New Home';   
public $minver = '5.1.5';   
public $maxver = '5.9.99';   
public $infourl = '';   
public $checkurl = '';   
public $AutoInstall = 0;   
public $extensionOrder = 5;   
public function fetchPageById($page, $pageid, $routeData = array()) {   
if(isset($_POST['page']))   
{   
echo "return";  
}   
page['hwWorld'] = $strReturn;   
return $page;   
}   
}  
?>

The ajax I used:

$(document).ready(function(){ 
$("a#pagetab").live("click",function(){

var fid=$(this).attr("rel"); 

var p=$(this).attr("data");
var rs = $.ajax({
url: "core/packages/helloworld/api/page.php",
type: "POST",
data: { forumid:fid, page:p },
dataType: "html",
beforeSend: function () {
$("div.active").html("<img src='modnews/images/gif-load.gif'/> Loading...");
}
});
rs.done(function(data){
$("div.active").html(data);
});
});});

And the result I got:

<br />
<b>Fatal error</b>: Class 'vB_Api_Extensions' not found in <b>/var/www/betawhitehat/core/packages/helloworld/api/page.php</b> on line <b>7</b><br />

Can help me solve this?
Thanks.