相同的代码，但不是两个中的一个？

You will have to get.class.php for giving help.

Working case(index.php spaghetti):

<?php
require_once 'classes/get.class.php';
$get = new Get();
$get->getSql("SELECT * FROM gunler ORDER BY g_id ASC LIMIT 0,15");
foreach($get->getData() as $data)
{
    echo $data["g_name"];
}
?>

result of spaghetti:

18 January 2012

the successful in a sense.

Are not working case:

<?php
require_once 'get.class.php';

Class Main Extends Get
{
    function __construct()
    {
        // boş
    }

    public function getGun($limit = 15, $sayfa = 1)
    {
        $limit1 = $limit * ($sayfa - 1);
        $limit2 = $limit1 + $limit;
        parent::getSql("SELECT * FROM gunler ORDER BY g_id ASC LIMIT " . $limit1 . "," . $limit2);
        return parent::getData();
    }
}

?>
$main = new Main();

print_r($main->getGun());

result of this:

Warning: mysql_query(): supplied argument is not a valid MySQL-Link resource in C:\AppServ\www\dailypremium\classes\get.class.php on line 39

Warning: mysql_fetch_assoc(): supplied argument is not a valid MySQL result resource in C:\AppServ\www\dailypremium\classes\get.class.php on line 55
Array ( )

I can't see any difference first codes between second codes. Where is the problem?

This is likely happening because you are not calling parent::__construct() in your overridden constructor and your database connection is never created.