Variable Recipient Email Form

I’m looking for a script–PHP is preferred–and I hope someone can point me in the right direction.

I’d like to create an online form that sends a personalized HTML page/email template to a SINGLE user (like a traditional email announcement list) based on the form recipient field.

The specific usage:
When a client renews an annual service, I want to send them an email thanking them for their business by way of the formatted/html email. However, I’m wanting to avoid an Outlook template solution since Outlook (and the template) won’t be on every machine needing access to this functionality. In other words, a web based solution.

Thanks in advance and I hope that made sense.