<?php
function parse_md5sum($filename) {
	$fp = fopen($filename, 'rt');

	$ln=0;
	while (($line = fgets($fp)) !== FALSE) {
		$ln++;
		if(!preg_match('/([a-fA-F0-9]{32})\s+([ *])(.*)/', $line, $matches)) {
			trigger_error("line $ln of md5sum file $filename doesn't match regex",E_USER_WARNING);
			continue;
		}
		$hashes[$matches[1]] = $matches[3];
	}
	fclose($fp);
	return $hashes;
}
function hash_part($fp,$start,$length) {
	$ctx = hash_init('md5');

	$remaining=$length;
	$chunkSize=65536;
	while ($remaining > 0) {
		$read = hash_update_stream($ctx, $fp, min($chunkSize, $remaining));
		//echo "$read\n";
		$remaining -= $read;
		//printf("\r%04.1f%%",(1-$remaining/$length)*100);
	}
	//echo "\n";
	return hash_final($ctx);
}
$hashes = parse_md5sum('sl-x86-3.4.loop1-to-3.4.loop2b.xd3.md5');
$fp = fopen('sl-x86-3.4.loop1-to-3.4.loop2b.xd3','rb');
$hash = hash_part($fp,0,40*1024*1024);
echo "$hash\n";
if (isset($hashes[$hash])) {
	$file = $hashes[$hash];
	echo "We have a match with file $file\n";
}
fclose($fp);
?>

Output: 
>php recover.php
40d99ed59fba1844207283d25f833c35
We have a match with file sl-x86-3.4.loop1-to-3.4.loop2b.xd3.00

>php recover.php
40d99ed59fba1844207283d25f833c35
We have a match with file sl-x86-3.4.loop1-to-3.4.loop2b.xd3.00

>php recover.php
40d99ed59fba1844207283d25f833c35
We have a match with file sl-x86-3.4.loop1-to-3.4.loop2b.xd3.00

>php recover.php
02621f7530933bab8dea50a61ff25003

>php recover.php
40d99ed59fba1844207283d25f833c35
We have a match with file sl-x86-3.4.loop1-to-3.4.loop2b.xd3.00