Reading large portions of files for hashing/checksums - php

Question

If I have three get parameters:
$filename = $_GET['filename'];
$start = $_GET['start'];
$size = $_GET['size'];
And I am reading a chunk of the file like so:
$handle = fopen($basepath . $filename, "rb");
fseek($handle, $start);
$contents = fread($handle, $size);
echo md5($contents);
How may I read large portions of a file (anywhere from 1mb to 1gb) and create a hash or checksum of its contents without needing to allocate enough memory for the entire read?
At the moment if I try to hash a too large of part of the file I get a memory error since php can not allocated enough memory (roughly 400mb).
Is there a hashing function in which I can digest parts of the file at a time rather than the entire contents at once (for example starting at $start read 100kb blocks and feed it to the function until $size is met)? And how would I read the file in chunks so that I would start at $start and read $size bytes?
If there is not such a hashing or checksum function that supports feeding chunks of the data pieces at a time, would file_get_contents() fix the issue of allocating memory for large reads? I am not entirely sure how that function works.
Thanks.

Answer 1

http://php.net/manual/en/function.hash-update.php
<?php
define('CHUNK', 65536);
//$file = 'alargefile.img';
//$start = 256 * 1024 * 1024;
//$size = 512 * 1024 * 1024;
$fp = fopen($file, "r");
fseek($fp, $start);
$ctx = hash_init('md5');
while ($size > 0) {
$buffer = fread($fp, min($size, CHUNK));
hash_update($ctx, $buffer);
$size -= CHUNK;
}
$hash = hash_final($ctx);
fclose($fp);
print $hash;
?>