Check if user is logged in

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<?php
if ( is_user_logged_in() ) {
    echo 'Welcome, registered user!';
} else {
    echo 'Welcome, visitor!';
};
?>

Instructions:

If you have users on your WordPress blog, you can choose to display stuff just for them.

Related links:

http://codex.wordpress.org/Function_Reference/is_user_logged_in