Secure PHP email form needed

Hi,

I'm starting out as a web designer. I have a couple of projects nearing completion which both require a secure contact form which will forward on a message to the clients' email address. Unfortunately I am relatively new to PHP and don't know how best to make a contact form secure. I have looked online, but no-one seems to agree on what level and what kind of security is necessary. The form will need to be the most secure they can possibly be as they are going to clients' email addresses and not mine, so any spam at all is not acceptable.
I would be very grateful and pay your hourly rate if you could either:
a) Provide a template secure contact form which I can adapt for each site by changing the contact email address;
or b) Provide advice/ lessons on the best and most secure way to do it myself.

Thanks for your help