<?php
function ConvertBytes($number){
$len = strlen ($number);
if ($len < 4){ return sprintf("%d b", $number); }
if ($len>= 4 && $len <=6){ return sprintf("%0.2f Kb", $number/1024); }
if ($len>= 7 && $len <=9){ return sprintf("%0.2f Mb", $number/1024/1024); }
return sprintf("%0.2f Gb", $number/1024/1024/1024);
}

ConvertBytes(intval($_GET['size']));

?>