PHP Contact Page Required With Security

Hello, a client of ours would like a contact form adding to their website. They would like the form to have a security method to stop spam bots and once the form is filled in the user would click send and this would forward to a 'thank you page'. The usual would be required, for example, not allowing a user to send the message without filling in the required details. Details required: Name, email address, Message. Also, with a drop-down that asked what department the question is in reference too and drop-down where they're from. This should be an easy job :-) Please PM me with details on your bid, but please don't send me a generic message with a huge list of domain names. Thank you. Jim