cron job won't write to file

Question

I run a windows 2003 server and tried to run a code like this every 15 minutes:

require("db_connect.php");
$conn = db_connect();

//list online brukere - flytt funksjon til separat side for bedre ytelse
$time = time() - 900;
$query ="SELECT username FROM tbl_user WHERE last_online >= $time";
$online_users; 
if ($result = $conn->query($query)) {
    while ($row = $result->fetch_assoc()) {
        $online_users .= $row["username"].":";
    }
    $result->close();
}   

$filename = "online_users.txt";
$fp = fopen($filename,"w");
fputs($fp,$online_users);
fclose($fp);

When I go to the url or run it from the command line it works and write to file. But the task just run and don't save the file.. What's wrong?

Answer 1

$filename = dirname(__FILE__) . "/online_users.txt";

Answer 2

$filename = "online_users.txt"; <-- this is using relative path

You might not have permission to write to relative path.

So, chose another path with enough write permission, and please, use absolute path instead.