function ByteSize ( $file_size )
{
	$file_size = $file_size-1;
	if ($file_size >= 1099511627776) $show_filesize = number_format(($file_size / 1099511627776),2) . " TB";
	elseif ($file_size >= 1073741824) $show_filesize = number_format(($file_size / 1073741824),2) . " GB";
	elseif ($file_size >= 1048576) $show_filesize = number_format(($file_size / 1048576),2) . " MB";
	elseif ($file_size >= 1024)  $show_filesize = number_format(($file_size / 1024),2) . " KB";
	elseif ($file_size > 0) $show_filesize = $file_size . " b";
	elseif ($file_size == 0 || $file_size == -1) $show_filesize = "0 b";
	return $show_filesize;
}

Human Readable filesize

PHP