mysql - PHP user session menu -

ok, i've searched site here alot, no avail, did not find solution, code have when user logged in display login menu, when not logged in display logged out menu:

<?php     session_start();     if(!$_session['myusername'])     {     echo '<a href="login.php">login</a> | <a href="register.php">register</a> | <a href="torrent.php">search</a>';     }     else     {     echo 'welcome <b>'; echo $_session["myusername"]; echo'</b> | <a href="torrent.php">search</a> | <a href="addtorrent.php">add torrent</a> | <a href="logout.php">logout</a>';     }     ?> 

but when logged out says: notice: undefined index: myusername

what can fix this? i've done many things.

<?php     session_start();     if(!isset($_session['myusername']))     {     echo '<a href="login.php">login</a> | <a href="register.php">register</a> | <a href="torrent.php">search</a>';     }     else     {     echo 'welcome <b>'; echo $_session["myusername"]; echo'</b> | <a href="torrent.php">search</a> | <a href="addtorrent.php">add torrent</a> | <a href="logout.php">logout</a>';     }     ?>