<?php

$username = 'test';
$email = 'test@test.com';
$str = "This is a test for $username. I am sending mail to some person $email.";

echo $str. "\n";
// This is a test for $username. I am sending mail to some person $email.
eval("\$str = \"$str\";");
echo $str. "\n";
// This is a test for test. I am sending mail to some person test@test.com.