jquery和.ajax的问题​​ - 空白响应

This is my setup.

I have a html registration form, when a user has typed in an username, email etc. The information is sent like this:

var processFile = "../inc/ajax.inc.php"; 

$("#register").change(function(){

    var postdata =  $("#register").serialize(); 

    $.ajax({
        type: "POST", 
        url: processFile, 
        data: "action=reg_validation&" + postdata , 
        success: function(data){                        
        }

    }); 

    console.log("action=reg_validation&" + postdata);   

}); 

This prints for example:

action=reg_validation&u=user&p=123&p2=&e=&r=

My ajax.inc.php file:

/*
 * Create a lookup array for form actions
 */
$actions = array(
        'reg_validation' => array(
                'object' => 'Intro',
                'method' => 'check_reg_form' 
        )
    );

if ( isset($actions[$_POST['action']]) )
{
    $use_array = $actions[$_POST['action']];   
    $obj = new $use_array['object'];

    echo $obj->$use_array['method'];      
}

function __autoload($class_name)
{
    $filename = '../../sys/class/class.'
        . strtolower($class_name) . '.inc.php';
    if ( file_exists($filename) )
    {
        include_once $filename;
    }
}

This code is supposed to call a class called Intro and a function called check_reg_form in class.intro.inc.php. This works almost.

If I start my class.intro.inc.php with:

echo "test 1"; 
exit(); 

I get test 1 in response, but this does not work:

class Intro
{
    public function check_reg_form()
    {
           echo "test 2"; 
    }
}

Not sure why this doesn't work... I get a blank response when I should get test 2..

I think you're just missing parens.

$obj = new $use_array['object']();      
echo $obj->$use_array['method']();