$dir = '/app/data';
    $file = $dir . "/test.txt";

    $fout = fopen( $file, "a");
    if (!$fout) {
        echo "Cannot create ".$file;
    } else {    
        $dt = date("Y.m.d H:i:s");
        fprintf($fout, "%s\n", $dt);
        fclose($fout);
        echo "file created ".$file;
    }