Question

When we use php setcookie() function, where are they created on client’s computer?

Answer 1

When you use setcookie(), an HTTP-header (Set-Cookie) is included, containing information about the cookie to be set, its expiration etc.

Where that cookie itself is stored is upto the browser.

The browser then sends the cookie’s content, which it stored somewhere, with the Cookie: header when appropriate, as described below.

There are two headers, Set-Cookie and Cookie,
that are related to cookies. The Set-Cookie header is sent by the
server in response to an HTTP request, which is used to create a
cookie on the user’s system. The Cookie header is included by the
client application with an HTTP request sent to a server, if there is
a cookie that has a matching domain and path.

RFC6265 defines the HTTP Cookie and Set-Cookie header fields.