Php Problem: SQL Variable im Script leer

<?php
include('C:\xamppneu\htdocs\test\zugang\admin\includes\controller.php');  
$form = new Form;
 echo $session->username; 
 
 
class Core {
    /* */

    private static $config_file = "conf.ini";  

    /* */
    //var $db_conf;

    private static $link = null;
    
    /**
     * 
     * @param type $file
     */
    public static function getDBConfig() {
        $conf = parse_ini_file(Core::$config_file);
        return $conf;
        //die(var_dump($conf) );
        //exit;
        //$db_conf = array();
        //$this->db_conf = $conf;
    }

    public static function connectDB() {
        $conf = Core::getDBConfig();
        //$link = mysql_connect($conf["host"], $conf["user"], $conf["password"]) or die("Could not connect: " . mysql_error()); 
/*$link = mysql_connect (MYSQL_HOST, 
                           MYSQL_BENUTZER, 
                           MYSQL_KENNWORT, 
                           MYSQL_DATENBANK);*/
        //Core::$link = new mysqli(MYSQL_HOST, MYSQL_BENUTZER, MYSQL_KENNWORT, MYSQL_DATENBANK);
        Core::$link = new mysqli($conf["host"], $conf["user"], $conf["password"], $conf["name"]);  
        //mysql_set_charset('utf8', $link); 
        /* check connection */
        if (mysqli_connect_errno()) {
            printf("Connect failed: %s\n", mysqli_connect_error());  
            exit();
        }
        if (!Core::$link->set_charset("utf8")) {  
            printf("Error loading character set utf8: %s\n", Core::$link->error);  
        } else {
            //printf("Current character set: %s\n", Core::$link->character_set_name()); 
        }
        
        // select our database
        
        // select our database
        //mysql_select_db(MYSQL_DATENBANK) or die(mysql_error());
        Core::$link->select_db($conf["name"]);  
    }

public static function getStaffList() {
		
		$sql = "SELECT * FROM staff where surname = 'Piet'";  
	
        $result = mysql_query($sql);

        if (!$result) {
            return 'Error: ' . mysqli_error(Core::$link);  
        }
        
        $return = array();
        while ($myrow = mysqli_fetch_assoc($result)) {
            if ($myrow['birthday'] !== '0000-00-00') {  
                $myrow['birthday'] = date("d.m.Y", strtotime($myrow['birthday']));  
            } else {
                $myrow['birthday'] = '';  
            }
            $return[] = $myrow;
        }

        //mysql_free_result($result);
        //$result->close();
        return $return;
    }