<?php
$input = "what do ya want for nothing?";
$hash = mhash(MHASH_MD5, $input);
echo "The hash is " . bin2hex($hash) . "<br />\n";
$hash = mhash(MHASH_MD5, $input, "Jefe");
echo "The hmac is " . bin2hex($hash) . "<br />\n";
?>
This will produce:
The hash is d03cb659cbf9192dcd066272249f8412
The hmac is 750c783e6ab0b503eaa86e310a5db738