leggere nomi files in cartella

[PaTeR]

ho provato così ma non funziona:

    $handle = 'f:/programmi/';
    while (false !== ($file = readdir($handle))) {
echo "$file\n";} 

errore:

//
Warning: readdir(): supplied argument is not a valid Directory resource in f:\programmi\apache group\Apache\localhost\i.php on line ...