How can PHP see client side cookies?

Question

How can PHP see client side cookies?

To elaborate: When working with PHP & Javascript, I understand that PHP gets processed on the server side. While Javascript happens on the client side.

In Javascript I can check and set these client cookies. That makes sense.

However, with PHP, if I check a client cookie value as part of a conditional statement that also sets , how is PHP able to see the clients cookie value while the PHP is happening on the server side?

Here's an example of the PHP conditional that lives in a php file:

<?PHP
if ($_COOKIE["name"] == “Mickey”) {
    setcookie(“fulsome”, “Mickey Mouse”, time()+3600);  
}
?>

Answer 1

There is no such thing as a client side cookie.

A cookie is a piece of data associated with a set of URLs in a browser. Every time the browser makes an HTTP request to one of those URLs, the cookie is included in the Request headers.

They were originally designed to be set via HTTP Response headers.

An API was added to browsers that allows them to be created and set by JavaScript. These are still regular cookies though and will be included in every request to the associated URLs.

It is possible to mark a cookie as http_only which will cause browsers to prevent access to that cookie from JavaScript. There is no direct equivalent for imposing a similar limit going the other way. The closest you could come to that would be to use something like Local Storage instead of cookies.

Answer 2

how is PHP able to see the clients cookie value while the PHP is happening on the server side?

The reason is that each request to server also bring cookies in headers like i have inspected in chorome network tab, my php website and got this:

Cookies are store on client side but are accessible on server side through request header. As @Quentin stated in his answer "Every time the browser makes an HTTP request to one of those URLs, the cookie is included in the Request headers."