(PHP 5 >= 5.1.2, PECL hash >= 1.1)
hash_hmac — Generate a keyed hash value using the HMAC method
$algo
   , string $data
   , string $key
   [, bool $raw_output = false
  ] )
algoName of selected hashing algorithm (i.e. "md5", "sha256", "haval160,4", etc..) See hash_algos() for a list of supported algorithms.
dataMessage to be hashed.
keyShared secret key used for generating the HMAC variant of the message digest.
raw_output
       When set to TRUE, outputs raw binary data.
       FALSE outputs lowercase hexits.
      
   Returns a string containing the calculated message digest as lowercase hexits
   unless raw_output is set to true in which case the raw
   binary representation of the message digest is returned.
   Returns FALSE when algo is unknown.
  
Example #1 hash_hmac() example
<?php
echo hash_hmac('ripemd160', 'The quick brown fox jumped over the lazy dog.', 'secret');
?>
The above example will output:
b8e7ae12510bdfb1812e463a7f086122cf37e4f7