Csv is not written to file [duplicate]

I don’t know what to put after the while. I have tried a million different ways and it does not write to the file.

if(isset($_POST['username']) && isset($_POST['password']) && isset($_POST['repassword'])){

        $Username = $_POST['username'];
        $Password = $_POST['password'];
        $RePassword = $_POST['repassword'];
        $users;

        $ficheroUsuarios = fopen("database/usuarios.csv", "r");
        $registros = array();

        if($ficheroUsuarios !== FALSE){   
            
            $nombres_campos = fgetcsv($ficheroUsuarios, 0, ",");
            $num_campos = count($nombres_campos);
            $numLinea = 1;
            while (($arrayLinea = fgetcsv($ficheroUsuarios, 1000, ",")) !== FALSE ){  

                
            }
            fclose($ficheroUsuarios);
        }
        else
        {
            echo 'No se ha podido leer el fichero';
        }
    }