You can create several parts like this:
$fileName = 'something_big.tar.gz';
$fileSize = filesize($fileName);
$parts = 10;
$partSize = ceil($fileSize / 10);
$f_in = fopen($fileName, 'rb');
for ($i = 0; $i < $parts; ++$i) {
$f_out = fopen("$fileName.$i", 'wb');
stream_copy_to_stream($f_in, $f_out, $partSize);
fclose($f_out);
}
fclose($f_in);