Format bytes to KiB, MiB, GiB and TiB.

format_size.php

<?php

if( ! function_exists('_format_size'))
{
    /**
     * Return formatted bytes.
     * 
     * @see http://www.iec.ch/si/binary.htm
     * @param  integer $bytes
     * @param  integer $kilo  1000, 1024
     * @return string
     */
    function _format_size($bytes, $kilo = 1000)
    {
        if(1024 == $kilo)
        {
            $m = [' bytes', 'KiB', 'MiB', 'GiB', 'TiB'];
        }

        else
        {
            $m = [' bytes', 'KB', 'MB', 'GB', 'TB'];
        }

        $s = array_reduce($m, function($a, $b) use($kilo) {
            if(is_numeric($a))
            {
                if($kilo <= $a)
                    return $a / $kilo;
                else
                    return number_format($a, 2) . $b;
            }
            else
                return $a;
        }, $bytes);

        return $s;
    }
}

$len = 10000000000;
print _format_size($len, 1000) . PHP_EOL; // 10.00GB
print _format_size($len, 1024) . PHP_EOL; // 9.31GiB