From http://www.if-not-true-then-false.com/2009/format-bytes-with-php-b-kb-mb-gb-tb-pb-eb-zb-yb-converter/

fomratBytes.php

<?php

/*
// Example usage:
echo byteFormat(4096, "B");
echo byteFormat(8, "B", 2);
echo byteFormat(1, "KB", 5);
echo byteFormat(1073741824, "B", 0);
echo byteFormat(1073741824, "KB", 0);
echo byteFormat(1073741824, "MB");
echo byteFormat(1073741824);
echo byteFormat(1073741824, "TB", 10);
echo byteFormat(1099511627776, "PB", 6);
*/

  function byteFormat($bytes, $unit = "", $decimals = 2) {
	$units = array('B' => 0, 'KB' => 1, 'MB' => 2, 'GB' => 3, 'TB' => 4, 
			'PB' => 5, 'EB' => 6, 'ZB' => 7, 'YB' => 8);

	$value = 0;
	if ($bytes > 0) {
		// Generate automatic prefix by bytes 
		// If wrong prefix given
		if (!array_key_exists($unit, $units)) {
			$pow = floor(log($bytes)/log(1024));
			$unit = array_search($pow, $units);
		}

		// Calculate byte value by prefix
		$value = ($bytes/pow(1024,floor($units[$unit])));
	}

	// If decimals is not numeric or decimals is less than 0 
	// then set default value
	if (!is_numeric($decimals) || $decimals < 0) {
		$decimals = 2;
	}

	// Format output
	return sprintf('%.' . $decimals . 'f '.$unit, $value);
  }


?>