Tweak the php code in my mailing form to increase security

I have a fully working php script that sends me emails. However, the script's security is non-existant. I would like for someone to increase the security through honeypot, captcha, preventing html/php code to be sent, preventing multiple quick submissions, etc. You can take a look at the code here: [login to view URL] Freelancers placing low bids and asking 'what's your budget' in the chat hoping for an increase will be blocked.