find and replace in php

Imagine I have a piece of text "Dear [firstname] Thankyou for purchasing [product] on [date]. We're sure you'll be very [expectation]. Ta Very much." Whats the easiest way to replace all the things in square brackets with variables of the same name. i.e. $firstname, $product, $date, $expectation Whilst ensuring that if the variable doesn't exist then it will be replaced with "" (absolutely nothing). Thankyou in advance Henry