php - "Allowed memory .. exhausted" when using readfile

Question

I made a download script in PHP that was working until yesterday. Today I tried to download one of the files only to discover that suddenly it stopped working:

PHP Fatal error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 119767041 bytes) in E:homeecnopontawebalunodownload.php on line 52

For some reason PHP is trying to allocate the size of the file in the memory, and I have no idea why. If the file size is smaller than the memory limit, I can download it without a problem, the problem is with bigger files.

I do know that it can be corrected by increasing the memory limit in php.ini or even use ini_set on the code but I would like a more accurate way to fix this and an answer to why it stopped working.

Here's my code:

$file = utf8_decode($_GET['d']);

header('Content-Description: File Transfer');
header('Content-Disposition: attachment; filename='.$file);
header('Content-Type: application/octet-stream');
header('Content-Transfer-Encoding: binary');
header('Expires: 0');
header('Cache-Control: must-revalidate');
header('Pragma: public');

$file = "uploads/$curso/$file";

ob_clean();
flush();
readfile($file);

echo "<script>window.history.back();</script>";
exit;

Answer 1

From php.net for readfile:

readfile() will not present any memory issues, even when sending large files, on its own. If you encounter an out of memory error ensure that output buffering is off with ob_get_level().

From php.net for ob_clean:

This function does not destroy the output buffer like ob_end_clean() does.

What you need is this:

if (ob_get_level()) {
      ob_end_clean();
    }

Also consider adding this:

header('Content-Length: ' . filesize($file));