For a non-looping way to add symbols to a number of bytes:
<?php
function getSymbolByQuantity($bytes) {
    $symbols = array('B', 'KiB', 'MiB', 'GiB', 'TiB', 'PiB', 'EiB', 'ZiB', 'YiB');
    $exp = floor(log($bytes)/log(1024));

    return sprintf('%.2f '.$symbol[$exp], ($bytes/pow(1024, floor($exp))));
}

Beware of empty files!

<?php

    // Wrong
    $exp = floor(log($bytes) / log(1024));

    //Correct
    $exp = $bytes ? floor(log($bytes) / log(1024)) : 0;

?>

"filesystem or disk partition" does not equal "directory" for Windows.  Thanks.

function roundsize($size){
    $i=0;
    $iec = array("B", "Kb", "Mb", "Gb", "Tb");
    while (($size/1024)>1) {
        $size=$size/1024;
        $i++;}
    return(round($size,1)." ".$iec[$i]);}

To find the total size of a file/directory you have to differ two situations:
(on Linux/Unix based systems only!?)

you are interested:
1) in the total size of the files in the dir/subdirs
2) what place on the disk your dir/subdirs/files uses

- 1) and 2) normaly differs, depending on the size of the inodes
- mostly 2) is greater than 1) (in the order of any kB)
- filesize($file) gives 1)
- "du -ab $file" gives 2)

so you have to choose your situation!

on my server I have no rights to use "exec du" in the case of 2), so I use:
  $s = stat($file);
  $size = $s[11]*$s[12]/8);
whitch is counting the inodes [12] times the size of them in Bits [11]

hopes this helps to count the used disk place in a right way... :-)

                     Andreas Dick