Código PHP:
<?php if(!isset($_SESSION["user_id"])):?>
<li class="nav_winning_numbers main_nav_item no_hover"><a class="main_nav_item" href="./register.php">REGISTRARME</a></li>
<li class="nav_winning_numbers main_nav_item no_hover"><a class="main_nav_item" href="./ingresar.php">INGRESAR</a></li>
<?php else:?>
<li class="nav_winning_numbers main_nav_item no_hover"><a class="main_nav_item" href="./php/logout.php"><font color="#010203">Cerrar Sesión</font></a></li>
<?php endif;?>