What potential issue did the user in the forum thread face when trying to limit the file size of an uploaded file using filesize()?

The potential issue the user faced when trying to limit the file size of an uploaded file using `filesize()` is that `filesize()` returns the size of the file on disk, not the size of the file being uploaded. To limit the size of the uploaded file, the user should use `$_FILES['file']['size']` instead.

if ($_FILES['file']['size'] > 1000000) {
    echo "File size must be less than 1MB.";
    exit;
}