Writing a new line to file in PHP (line feed)

My code:

$i = 0;
$file = fopen('ids.txt', 'w');
foreach ($gemList as $gem)
{
    fwrite($file, $gem->getAttribute('id') . '\n');
    $gemIDs[$i] = $gem->getAttribute('id');
    $i++;
}
fclose($file);

For some reason, it's writing \n as a string, so the file looks like this:

40119\n40122\n40120\n42155\n36925\n45881\n42145\n45880

From Google'ing it tells me to use \r\n, but \r is a carriage return which doesn't seem to be what I want to do. I just want the file to look like this:

40119
40122
40120
42155
36925
45881
42145
45880

Thanks.