为什么我不能将$this与get_object_vars一起使用

<?php 
     require_once 'database.php';
class User {
public $id;
public $username;
public $first_name;
public $last_name;
public $password;
public static function find_by_id($id){
    $result_array = self::find_by_sql("SELECT * FROM users WHERE id = {$id} LIMIT 1");
    return !(empty($result_array))? array_shift($result_array): false;
}
public static function find_by_sql($sql){
    global $database;
    $result = $database->query($sql);
    $object = array();
    while ($row = $database->fetct_array($result)){
     $object[] = self::instantiate($row);  
    }
    return $object;
}
public static function instantiate($record)
        {
    $object = new self;
    foreach($record as $attribute => $value)
        {
        if ($object->has_attribute($attribute)){
            $object->$attribute = $value;
        }
    }
    return $object;
}
private static function has_attribute($attribute){       
    $object_var = get_object_vars($this);
    return array_key_exists($attribute, $object_var);

}
}
?>

尝试调用函数 has_attribute 时出错。注意：未定义的变量：这个

为什么我不能在私人函数has_attribute($attribute) get_object_vars使用它.谁能帮我？谢谢。