Get Contents from Text File and Replace Keywords PHP - php

Question

I have a form. I'm using "post" method that sends and emails to multiple people that registered. Then the $body of the email is based on a template. No database, no classes, simple form. Yes I read up about this already they're all there I just couldn't put it together, related to this case.
The text "email_template.txt" should have something like:
Hello #firstname# #lastname#. Thank you for registering! Your registered email is #email#
Would look like this upon processing by PHP
Hello **John Doe**. Thank you registering! Your registered email is **example#example.com**.
On my php I have something like:
<?php
//form and validation
$firstname = "John"; // inputed name on the form, let say
$lastname = "Doe"; // inputed last name
$email = "example"; // inputed email
//email message
$body = ... some type of a get_file_content....
mail($_POST['email'], 'Party Invitation', $body, 'From: admin#admin.com');
?>
Where $body is the email message to the registrants submitted via this PHP form.

Answer 1

sprintf will work good
your template could be:
Hello %s %s. Thank you for registering! Your registered email is %s
$body = sprintf($fileContents,$firstname,$lastname,$email);
or str_replace(), pretty much a find replace.
$body = "Hello #firstname# #lastname#. Thank you for registering! Your registered email is #email#";
$body = str_replace("#firstname#",$firstname,$body);
$body = str_replace("#lastname#",$lastname,$body);
$body = str_replace("#email#",$email,$body);