Try like
HTML in PHP :
echo "<a href="".$link_address."">Link</a>";
Or even you can try like
echo "<a href="https://stackoverflow.com/questions/18916966/$link_address">Link</a>";
Or you can use PHP in HTML like
PHP in HTML :
<a href="https://stackoverflow.com/questions/18916966/<?php echo $link_address;?>"> Link </a>