I'm including a file in one of my class methods, and in that file has html + php code. I return a string in that code. I explicitly wrote {{newsletter}} and then in my method I did the following:

$contactStr = include 'templates/contact.php';
$contactStr = str_replace("{{newsletter}}",$newsletterStr,$contactStr);

However, it's not replacing the string. The only reason I'm doing this is because when I try to pass the variable to the included file it doesn't seem to recognize it.

$newsletterStr = 'some value';
$contactStr = include 'templates/contact.php';

So, how do I implement the string replacement method?