Problem with filemtime

I'm running the following script (simplified for discussion) to create a CSV file. My problem here is that the first filemtime and the last filemtime always equal each other. What I am trying to get is the filemtime ($new_last_modified) of the file that was just executed 3 lines earlier. Is there something I need to do differently? I tried putting a sleep(20) after the fclose, but that didn't seem to help.

$csv_file = "myFile.csv";
$last_modified_csv = filemtime($csv_file);

$fh = fopen($csv_file, 'w') or die("can't open file");
fwrite($fh, "Data Goes Here");
fclose($fh);

$new_last_modified = filemtime($csv_file);

--
Kevin Murphy
Webmaster: Information and Marketing Services
Western Nevada College
www.wnc.edu
775-445-3326