How cookies work in PHP - Visual Walkthrough

Concept Flow - How cookies work

Client sends HTTP request

↓

Server checks for cookies

↓

Server sends HTTP response with Set-Cookie header

↓

Browser stores cookie

↓

Browser sends cookie with next requests

↓

Server reads cookie from request

↓

Repeat cycle

This flow shows how a browser and server exchange cookies through HTTP requests and responses.

Execution Sample

PHP

<?php
// Set a cookie
setcookie('user', 'Alice', time() + 3600);

// Read cookie
if (isset($_COOKIE['user'])) {
    echo 'Hello ' . $_COOKIE['user'];
}
?>

This PHP code sets a cookie named 'user' and then reads it to greet the user.

Execution Table

Step	Action	Server Behavior	Browser Behavior	Output
1	Client sends first HTTP request	No cookie received	No cookie stored yet	No output
2	Server runs setcookie('user', 'Alice', time()+3600)	Sends Set-Cookie header: user=Alice; expires=+1 hour	Receives response with Set-Cookie header	No output yet
3	Browser stores cookie 'user=Alice'	No action	Stores cookie for domain	No output
4	Client sends second HTTP request	Receives cookie 'user=Alice' in request headers	Sends cookie with request	No output
5	Server checks $_COOKIE['user']	Finds 'user' cookie with value 'Alice'	No action	Outputs: Hello Alice
6	End of script	Response sent with greeting	Displays greeting	Hello Alice

💡 Execution stops after server sends greeting and browser displays it.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
$_COOKIE['user']	undefined	undefined	undefined	'Alice'	'Alice'	'Alice'
Cookie stored in browser	none	none	'user=Alice'	'user=Alice'	'user=Alice'	'user=Alice'

Key Moments - 2 Insights

Why doesn't the cookie appear in $_COOKIE immediately after setcookie()?

Because setcookie() only sends a header to the browser; the cookie is stored on the client and sent back on the next request, as shown between steps 2 and 4 in the execution_table.

How does the browser know to send the cookie back to the server?

The browser stores the cookie after receiving the Set-Cookie header (step 3) and automatically includes it in the headers of subsequent requests (step 4).

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the server first receive the cookie from the browser?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Check the 'Server Behavior' column to see when the cookie is received in the request headers.

According to variable_tracker, what is the value of $_COOKIE['user'] after the first request?

A Alice

B null

C undefined

D empty string

💡 Hint

Look at the 'After Step 2' column for $_COOKIE['user'] in variable_tracker.

If the cookie expiration time is set to time() - 3600, what would happen in the execution flow?

A Browser deletes the cookie immediately

B Browser stores cookie normally

C Server ignores the cookie

D Cookie value changes to empty string

💡 Hint

Recall that setting expiration in the past causes the browser to delete the cookie.

Concept Snapshot

PHP cookies are small data stored on the browser.
Use setcookie() to send cookies from server to browser.
Browser stores cookies and sends them back with requests.
$_COOKIE array holds cookies sent by browser.
Cookies appear in $_COOKIE only on next request after setcookie().

Full Transcript

This visual trace shows how cookies work in PHP. First, the client sends a request without cookies. The server uses setcookie() to send a cookie in the response header. The browser stores this cookie and sends it back with the next request. The server reads the cookie from $_COOKIE and can use it, for example, to greet the user. Cookies are stored on the client and only appear in $_COOKIE on subsequent requests after being set. This cycle repeats with each request and response.