speed.php

<?php

session_start();

$si_prefix = array( 'B', 'KB', 'MB', 'GB', 'TB', 'EB', 'ZB', 'YB' );
$base = 1024;

if( !is_array($_SESSION['speed']) )
{
   $_SESSION['speed'] = array('updated' => time(), 'old' => file_get_contents('/sys/class/net/eth0/statistics/rx_bytes'));
   echo json_encode(array('speed' => '... B/s'));
   die();
}

$old = $_SESSION['speed']['old'];
$elapsed = time() - $_SESSION['speed']['updated'];
$current = file_get_contents('/sys/class/net/eth0/statistics/rx_bytes');
$difference = $current - $old;
$diff_per_second = $difference / $elapsed;

$speed = min((int)log($diff_per_second, $base), count($si_prefix) - 1);
$speed = sprintf('%1.2f', $diff_per_second / pow($base, $speed)) . ' ' . $si_prefix[$speed] . '/s';
$_SESSION['speed']['updated'] = time();
$_SESSION['speed']['old'] = file_get_contents('/sys/class/net/eth0/statistics/rx_bytes');

echo json_encode(array('speed' => $speed));
die();