Adding a string to a session variable echo

Sorry, struggling to find this on here, probably because its really simple. Just want to add a string to this code. Such as "Welcome: +$SESSION".

New to PHP dont really know how to add the extra line within this code.

<label class="welcome"><?PHP if(isset($_SESSION['u_username'])){echo $_SESSION['u_username'];}?></label>

#html #php #string