I have a .php that contents the body of a text email.
In that body I want to create a link that when the user receives the email can be clicked.
How to create that link?
If it is as simple as your question sounds then..
<?php echo '<a href="http://yourdomain.com/link.html">Click Here</a>'; ?>
a tag will do them job.
This is assuming that youu are using a HTML based email, if you are using plain text then I would suggest just typing in the URL