This question would probably have been better asked in the programming part of the forum. but never mind, I'll try and help...
The easy answer:
<input .... autocomplete='off' ...>
The good news is that the autocomplete='off' attribute will do pretty much exactly what you're asking for.
The bad news is that it's a non-standard attribute. So if you use it, you can't claim that your site is standards-compliant, and you can't guarantee that it'll work in all browsers (as far as I'm aware, it does work in MSIE and Firefox, but I can't vouch for any others, and I can't vouch for it working in the future).
If that bad news makes it a non-starter, then the next best suggestion is to have php generate a unique name for the input field each time you display the form.
Hope that helps.