How to set and read a cookie

Short answer

setcookie() sends a cookie, and the $_COOKIE array reads them on the next request:

<?php
 
setcookie('theme', 'dark', [
    'expires' => time() + 60 * 60 * 24 * 30,
    'path' => '/',
    'httponly' => true,
    'samesite' => 'Lax',
]);
 
$theme = $_COOKIE['theme'] ?? 'light';

Removing

Send a cookie of the same name with a date in the past:

<?php
 
setcookie('theme', '', ['expires' => time() - 3600, 'path' => '/']);

Two rules

Call setcookie() before any output to the page. And a cookie cannot be trusted: the visitor can change the value, so rights and prices are not kept in them.