HTML page with PHP "fields"

I have been wondering how to do this properly for a while.

Let’s say I have a page which displays a letter to the user with various fields filled in

Good morning, Mr/Mrs [$data->name]!
We have received your question regarding our [$data->productName]. Our representative should contact you within [$data->contactDays] days, or your money back. Thank you from [$data->companyName].

Now, in addition, imagine that page being several paragraphs long and/or filled with HTML formatting brackets.

I like to separate off my PHP code from the HTML code to make it easier on the designer (who is a different person), yet still keep these “fields” that are easy to fill in.

How can this be done easily and efficiently?