<?php
function hash_part($fp,$start,$length) {
	$ctx = hash_init('md5');

	$remaining=$length;
	$chunkSize=65536;
	while ($remaining > 0) {
		$read = hash_update_stream($ctx, $fp, $chunkSize);
		if($read != $chunkSize) {
			echo "Unexpected read value $read, when we had $remaining bytes left\n";
			echo "We ".(feof($fp)?'ARE':'are NOT')." in EOF\n";
		}
		$remaining -= $read;
		//printf("\r%04.1f%%",(1-$remaining/$length)*100);
	}
	//echo "\n";
	return hash_final($ctx);
}
//$fp = fopen('sl-x86-3.4.loop1-to-3.4.loop2b.xd3','rb');
$fp = fopen('../../directx_aug2006_redist.exe','rb');
$hash = hash_part($fp,0,40*1024*1024);
echo "$hash\n";
if ($hash != "64468f8a359ab1dfcb770c2f81d60fd0") {
	echo "WHAT THE HELL IS GOING ON\n";
}
fclose($fp);
?>
Output on ten runs:
68d6029b53aa809b7668b08f2052addf
WHAT THE HELL IS GOING ON
64468f8a359ab1dfcb770c2f81d60fd0
597403d419936ef0f425b7cf30303e28
WHAT THE HELL IS GOING ON
64468f8a359ab1dfcb770c2f81d60fd0
64468f8a359ab1dfcb770c2f81d60fd0
64468f8a359ab1dfcb770c2f81d60fd0
64468f8a359ab1dfcb770c2f81d60fd0
64468f8a359ab1dfcb770c2f81d60fd0
04b48a6570d4bb3d39c2c8f32de36bca
WHAT THE HELL IS GOING ON
64468f8a359ab1dfcb770c2f81d60fd0