How about that:
function convert_size($from,$to,$unit){
$units = array('B', 'KB', 'MB', 'GB', 'TB','PB','EB','ZB','YB');
if(($fromPosition = array_search($from,$units)) === false){
return false;
}
if(($toPosition = array_search($to,$units)) === false){
return false;
}
$diffUnits = $fromPosition-$toPosition;
return $unit * (pow(1024, $diffUnits));
}