为什么不能在ob_start函数的回调中使用类对象？

<?PHP
$f=new stdClass();
$f->title="awesome Title";

function callback($buffer) 
{
    global $f;
    $buffer=str_replace("###TITLE###",$f->title,$buffer);
    return $buffer;
}
ob_start("callback");
?>

This is the ###TITLE###

输出为：

PHP Notice:  Trying to get property of non-object in /Users/qxxx/Sites/test/test.PHP on line 8
This is the

应该：

This is the awesome Title