Calculates the MD5 hash of the specified filename using the RSA Data Security, Inc. MD5 Message-Digest Algorithm, and returns that hash. The hash is a 32-character hexadecimal number. If the optional raw_output is set to TRUE, then the md5 digest is instead returned in raw binary format with a length of 16.
Nota: The optional raw_output parameter was added in PHP 5.0.0 and defaults to FALSE
This function has the same purpose of the command line utility md5sum.