Format a number of bytes.

format_memory.php

<?php

/**
 * Format a number of bytes.
 *
 * Format a number of bytes into a readable string in either IETF (kibibyte,
 * 1024 bytes) or SI (kilobyte, 1000 bytes) units.
 * Produces output like "1 byte", "28 bytes", "4 MB" or "22 GiB".
 *
 * @param $memory	The number of bytes to format. The result of 
 * 			{@link http://php.net/memory_get_usage memory_get_usage} 
 * 			is used if omitted.
 * @param $precision	The number of decimal places to display. 0 if omitted.
 * @param $useSiUnits	Whether to use SI units or not. IETF units by default.
 *
 * @author Craig Anderson <craiga@craiga.id.au>
 * @link http://gist.github.com/2880386
 */
function format_memory($memory = null, $precision = 0, $useSiUnits = false)
{
	if(is_null($memory))
	{
		$memory = memory_get_usage();
	}
	$result = null;
	if($memory == 1)
	{
		$result = "1 byte";
	}
	else
	{
		$unit = "bytes";
		if($useSiUnits) {
			if(abs($memory) > 1999)
			{
				$memory = $memory / 1000;
				$unit = "KB";
			}
			if(abs($memory) > 1999)
			{
				$memory = $memory / 1000;
				$unit = "MB";
			}
			if(abs($memory) > 1999)
			{
				$memory = $memory / 1000;
				$unit = "GB";
			}
		}
		else {
			if(abs($memory) > 2047)
			{
				$memory = $memory / 1024;
				$unit = "KiB";
			}
			if(abs($memory) > 2047)
			{
				$memory = $memory / 1024;
				$unit = "MiB";
			}
			if(abs($memory) > 2047)
			{
				$memory = $memory / 1024;
				$unit = "GiB";
			}
		}
		$result = sprintf("%s %s", number_format($memory, $precision), $unit);
	}
	return $result;
}

test.php

<?php

require_once("format_memory.php");

$numByteses = array(-1, 0, 1, 999, 1000, 1001, 1023, 1024, 1025, 2047, 2048, 2049, 3000000, 3000001, 3145727, 3145728, 3145729);

printf("Num Bytes  SI                   Non-SI\n");
foreach($numByteses as $key => $numBytes) {
	printf("%-10s %-20s %s\n", number_format($numBytes), format_memory($numBytes, 3, true), format_memory($numBytes, 3, false));
}