Check user is logged in or not

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

From your functions file, this code displays a personal message for logged in users.
 

add_action( 'loop_start', 'personal_message_when_logged_in' );

function personal_message_when_logged_in() {

if ( is_user_logged_in() ) :
 
    $current_user = wp_get_current_user();

    echo 'Personal Message For ' . $current_user->user_firstname . '!';

else :
        echo 'Non Personalized Message!';

    endif;
}