Checksum
A checksum is a small, fixed-size value derived from a longer piece of data, used to detect errors or verify integrity. It is akin to a fingerprint or a signature.
There are several algorithms to create a checksum: crc32, md5, sha1, sha256.
Checksums are extensively used to check file downloads and network transmissions; to verify stored data, identify revisions in VCS and validate components versions with package managers.
<?php
echo md5('PHP rocks!');
echo md5_file('/tmp/text.txt');
?>