Call to undefined method Data::array_keys()

private static $columns = [];

public function Create($table){
    self::$columns=(new Data)->array_keys(Data::Select('*', "$table"));
    foreach (self::$columns as $this->column){
        $this->column = $this->mysqli->real_escape_string($this->column);
        if(self::$columns[$this->column]!=null) {
            $a = "'" . $this->column . "'=>" . $_POST[$this->column];
        }
        else{
            $a = "'" . $this->column . "'=>" . null;
        }
        echo $a;
    }
}

Вызываю метод SELECT из другого класса Data.