How to set a cookie using PHP in WordPress

set-cookie-wordpress.php

<?php
/**
 * Sets a cookie sitename_new_visitor if it doesn't exist already.
 */
function example_set_new_user_cookie() {
	if ( ! is_admin() && ! isset( $_COOKIE['sitename_new_visitor'] ) ) {
		setcookie( 'sitename_new_visitor', 1, time() + 3600 * 24 * 100, COOKIEPATH, COOKIE_DOMAIN, false );
	}
}
add_action( 'init', 'example_set_new_user_cookie');

/**
 * Displays a different message based on whether the sitename_new_visitor cookie is set or not
 */
if ( isset( $_COOKIE['sitename_new_visitor'] ) ) {
     echo 'Welcome back!';
}
else {
     echo 'Hello new visitor!';
}