Why is the equal sign in the cookie being replaced with %3D?

Question

PHP

I'm trying to set a cookie:

// eyJhbGciOiJzaGEyNTYiLCJ0eXAiOiJKV1QifQ%3D%3D.{$payload_refresh}.{$signature}
setcookie('token', $refresh_token, $payload_refresh->exp, '/'auth, 'site.ru', true, true);

When I set a cookie, I get a double equal at the end of the line.
And it is replaced by a double one - %3D.
Question: Is this how it should be or is something going wrong? And if it goes wrong, how to fix the situation?

IMHO: it looks like the equal sign is reserved, because it is used to assign a value to some field.