How sessions work in PHP - Visual Walkthrough

Concept Flow - How sessions work in PHP

User visits page

↓

Check if session exists

↓

Use session

↓

Store/Retrieve data in $_SESSION

↓

Send session ID cookie to browser

↓

User makes next request with session ID

↩Back to Check if session exists

This flow shows how PHP checks for a session, starts one if needed, stores data, and uses a session ID cookie to keep track across requests.

Execution Sample

PHP

<?php
session_start();
$_SESSION['user'] = 'Alice';
echo $_SESSION['user'];
?>

Starts a session, saves 'Alice' in session data under 'user', then prints it.

Execution Table

Step	Action	Session Status	$_SESSION Content	Output
1	Call session_start()	No session -> New session created	{}
2	Assign 'Alice' to $_SESSION['user']	Session active	{'user': 'Alice'}
3	Echo $_SESSION['user']	Session active	{'user': 'Alice'}	Alice
4	End of script	Session active	{'user': 'Alice'}	Alice

💡 Script ends, session data saved on server, session ID sent to browser cookie

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
$_SESSION	undefined	{}	{'user': 'Alice'}	{'user': 'Alice'}	{'user': 'Alice'}

Key Moments - 3 Insights

Why do we need to call session_start() at the beginning?

session_start() initializes or resumes a session so $_SESSION can be used. Without it, $_SESSION is empty (see Step 1 in execution_table).

How does PHP remember the session between page loads?

PHP sends a session ID cookie to the browser (explained in concept_flow). The browser sends it back on next requests to link to the saved session data.

What happens if we don't store data in $_SESSION?

The session exists but no data is saved. $_SESSION stays empty, so no user info or state is remembered (see Step 1 and 2).

Visual Quiz - 3 Questions

Look at the execution_table, what is the content of $_SESSION after Step 2?

A {}

B {'user': 'Alice'}

C undefined

D {'user': ''}

💡 Hint

Check the '$_SESSION Content' column at Step 2 in execution_table

At which step does the session get created?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Session Status' column in execution_table for Step 1

If we remove session_start(), what will be the output at Step 3?

A Error or empty output

B Alice

C Session ID

D PHP warning but still outputs Alice

💡 Hint

Without session_start(), $_SESSION is not initialized (see key_moments about session_start)

Concept Snapshot

PHP sessions track user data across pages.
Call session_start() first to begin or resume a session.
Use $_SESSION superglobal to store/retrieve data.
PHP sends a session ID cookie to browser.
Browser sends it back to link requests to session data.
Without session_start(), $_SESSION is empty.

Full Transcript

This visual trace shows how PHP sessions work step-by-step. When a user visits a page, PHP checks if a session exists by looking for a session ID cookie. If none exists, session_start() creates a new session. We can then store data in the $_SESSION array, like saving the username 'Alice'. PHP sends a session ID cookie to the browser, which returns it on the next request to continue the session. The execution table shows each step: starting the session, assigning data, and outputting it. The variable tracker follows $_SESSION's content as it changes. Key moments clarify why session_start() is needed, how PHP remembers sessions, and what happens if no data is stored. The quiz tests understanding of session creation, $_SESSION content, and the importance of session_start(). This helps beginners see how PHP keeps user data across page loads using sessions.