How to Secure Contact Form Script in PHP?

Good Day Friends!
Anyone please help? How to secure contact form script for my website? There are available sample scripts in the web but I want to make sure that it’s secure and safe. So, I want to consult the experts and guide me on creating a secure contact form? What are the things that I need to consider first? What are the tools that I can use? What techniques I should use to create a secure and safe contact form? How will I know that my contact form script is safe and secure? Is there a checker or application that will try the security of my script? I need your comments and suggestions please.
Thank you,
Alex
