key: wordpress_logged_in_1234567890abcdef1234567890abcdef
value: admin|1234567890|abcdef1234567890abcdef1234567890

$key = ... // the key from the WP cookie
$value = ... // the value from the WP cookie
$hash = hash_hmac ( 'md5' , $key.$value , 'some secret key' );