You can create a simple logout-link like this:

<?php 
    $user = JFactory::getUser();
    $userToken = JSession::getFormToken();
    if (!$user->guest) : ?>
        <a href="index.php?option=com_users&task=user.logout&<?php echo $userToken; ?>=1" class="btn">Log out</a>
<?php endif; ?>

The Log out button will only be visible to logged-in users, and when clicked the user will be logged out instantly. If you also want to show a Login button, you can modify the PHP if/else statement and display a link to the login page (index.php?option=com_users&view=login) for guests.

<?php 
    $user = JFactory::getUser();
    $userToken = JSession::getFormToken();
    if ($user->guest) : ?>
        <a href="index.php?option=com_users&view=login" class="btn">Log in</a>
<?php else: ?>
        <a href="index.php?option=com_users&task=user.logout&<?php echo $userToken; ?>=1" class="btn">Log out</a>
<?php endif; ?>

The combination of PHP and HTML is necessary because we need to get the user token, without it the link will fail.

##Usage:##

The code can be used in several ways:

1. Custom HTML
   Some Custom HTML modules accept PHP code (the one included in Joomla by default does not). I recommend Custom HTML Advanced. Just paste the code and publish the module wherever you want the button to show up. Remember you can also place a module anywhere using {loadposition YourModulePosition}.

2. Sourcerer
   You can add PHP code anywhere using NoNumbers Sourcerer.

    {source}
    <?php 
        $user = JFactory::getUser();
        $userToken = JSession::getFormToken();
        if (!$user->guest) : ?>
            <a href="index.php?option=com_users&task=user.logout&<?php echo $userToken; ?>=1" class="btn">Log out</a>
    <?php endif; ?>
    {/source}
   

3. Build a module
   You can build your own module, it's easier than it sounds. A basic tutorial can be found here.

4. Add to your template
   You can also paste the code directly into your Joomla template, usually ROOT\templates\YOURTEMPLATE\index.php. This is the least flexible solution, but might be a quick solution depending on how your template is configured.

You can create a simple logout-link like this:

<?php 
    $user = JFactory::getUser();
    $userToken = JSession::getFormToken();
    if (!$user->guest) : ?>
        <a href="index.php?option=com_users&task=user.logout&<?php echo $userToken; ?>=1" class="btn">Log out</a>
<?php endif; ?>

The Log out button will only be visible to logged-in users, and when clicked the user will be logged out instantly. If you also want to show a Login button, you can modify the PHP if/else statement and display a link to the login page (index.php?option=com_users&view=login) for guests.

<?php 
    $user = JFactory::getUser();
    $userToken = JSession::getFormToken();
    if ($user->guest) : ?>
        <a href="index.php?option=com_users&view=login" class="btn">Log in</a>
<?php else: ?>
        <a href="index.php?option=com_users&task=user.logout&<?php echo $userToken; ?>=1" class="btn">Log out</a>
<?php endif; ?>

The combination of PHP and HTML is necessary because we need to get the user token, without it the link will fail.

##Usage:##

The code can be used in several ways:

1. Download from GitHub
   This module is available on GitHub as an installable Joomla module: https://github.com/Joomla-StackExchange/loginButton

2. Custom HTML
   Some Custom HTML modules accept PHP code (the one included in Joomla by default does not). I recommend Custom HTML Advanced. Just paste the code and publish the module wherever you want the button to show up. Remember you can also place a module anywhere using {loadposition YourModulePosition}.

3. Sourcerer
   You can add PHP code anywhere using NoNumbers Sourcerer.

    {source}
    <?php 
        $user = JFactory::getUser();
        $userToken = JSession::getFormToken();
        if (!$user->guest) : ?>
            <a href="index.php?option=com_users&task=user.logout&<?php echo $userToken; ?>=1" class="btn">Log out</a>
    <?php endif; ?>
    {/source}
   

4. Build a module
   You can build your own module, it's easier than it sounds. A basic tutorial can be found here.

5. Add to your template
   You can also paste the code directly into your Joomla template, usually ROOT\templates\YOURTEMPLATE\index.php. This is the least flexible solution, but might be a quick solution depending on how your template is configured.

You can create a simple logout-link like this:

<?php 
    $user = JFactory::getUser();
    $userToken = JSession::getFormToken();
    if (!$user->guest) : ?>
        <a href="index.php?option=com_users&task=user.logout&<?php echo $userToken; ?>=1" class="btn">Log out</a>
<?php endif; ?>

The Log out button will only be visible to logged-in users, and when clicked the user will be logged out instantly. If you also want to show a Login button, you can modify the PHP if/else statement and display a link to the login page (index.php?option=com_users&view=login) for guests.

<?php 
    $user = JFactory::getUser();
    $userToken = JSession::getFormToken();
    if ($user->guest) : ?>
        <a href="index.php?option=com_users&view=login" class="btn">Log in</a>
<?php else: ?>
        <a href="index.php?option=com_users&task=user.logout&<?php echo $userToken; ?>=1" class="btn">Log out</a>
<?php endif; ?>

You can create a simple logout-link like this:

<?php 
    $user = JFactory::getUser();
    $userToken = JSession::getFormToken();
    if (!$user->guest) : ?>
        <a href="index.php?option=com_users&task=user.logout&<?php echo $userToken; ?>=1" class="btn">Log out</a>
<?php endif; ?>

The Log out button will only be visible to logged-in users, and when clicked the user will be logged out instantly. If you also want to show a Login button, you can modify the PHP if/else statement and display a link to the login page (index.php?option=com_users&view=login) for guests.

<?php 
    $user = JFactory::getUser();
    $userToken = JSession::getFormToken();
    if ($user->guest) : ?>
        <a href="index.php?option=com_users&view=login" class="btn">Log in</a>
<?php else: ?>
        <a href="index.php?option=com_users&task=user.logout&<?php echo $userToken; ?>=1" class="btn">Log out</a>
<?php endif; ?>

The combination of PHP and HTML is necessary because we need to get the user token, without it the link will fail.

##Usage:##

The code can be used in several ways:

1. Custom HTML
   Some Custom HTML modules accept PHP code (the one included in Joomla by default does not). I recommend Custom HTML Advanced. Just paste the code and publish the module wherever you want the button to show up. Remember you can also place a module anywhere using {loadposition YourModulePosition}.

2. Sourcerer
   You can add PHP code anywhere using NoNumbers Sourcerer.

    {source}
    <?php 
        $user = JFactory::getUser();
        $userToken = JSession::getFormToken();
        if (!$user->guest) : ?>
            <a href="index.php?option=com_users&task=user.logout&<?php echo $userToken; ?>=1" class="btn">Log out</a>
    <?php endif; ?>
    {/source}
   

3. Build a module
   You can build your own module, it's easier than it sounds. A basic tutorial can be found here.

4. Add to your template
   You can also paste the code directly into your Joomla template, usually ROOT\templates\YOURTEMPLATE\index.php. This is the least flexible solution, but might be a quick solution depending on how your template is configured.