OAuthException: Invalid OAuth access token signature - php

Question

When i type this url direct into browser i get the post sent to user feed wall
https://graph.facebook.com/xx facebook uid xx/feed?access_token=xx app access token xx&message=Welcome%20to%20App!&method=post
But it gives me this error OAuthException: Invalid OAuth access token signature. when i automate the post with this code
<?PHP
include_once "inc/facebook.php";
require_once 'inc/config.php';
// Create our application instance
$facebook = new Facebook(array(
'appId' => $app_id,
'secret' => $app_secret,
));
$dbh = new PDO('mysql:dbname='.$db_name.';host='.$db_host.';charset=utf8', $db_username, $db_password );
$dbh->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
$dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$stmt = $dbh->prepare('SELECT fb_id FROM offline_access_users');
$stmt->execute();
$body = array(
'message' => 'Test Multiple Messages',
'link' => '',
'picture' => '',
'name' => '',
'description'=> ''
);
$batchPost=array();
$i=1;
foreach ($stmt as $value) {
$id = $value['fb_id'];
$batchPost[] = array(
'method' => 'POST',
'relative_url' => "/" . $id . "/feed?access_token=" . $access_token,
'body' => http_build_query($body) );
if($i++ == 50) {
try {
$multiPostResponse = $facebook->api('?batch=' . urlencode(json_encode($batchPost)), 'POST');
echo "Success";
} catch(FacebookApiException $e) {
error_log($e);
echo($e);
}
unset($batchPost);
$i=1;
}
}
if(isset($batchPost) && count($batchPost) > 0 ) {
try{
$multiPostResponse = $facebook->api('?batch=' . urlencode(json_encode($batchPost)), 'POST');
echo "Success";
} catch(FacebookApiException $e){
error_log($e);
echo($e);
}
}
$db = null;
?>
I really don't know how to come over this issue. Please help me