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.

Practice

1. What is the main purpose of using sessions in PHP?

easy

A. To remember user data across multiple pages

B. To store data permanently on the user's computer

C. To speed up the website loading time

D. To create a new database connection

Solution

1. Step 1: Understand what sessions do

   Sessions keep track of user information as they move between pages on a website.

2. Step 2: Compare options

   Only To remember user data across multiple pages correctly describes this purpose. Options A, C, and D describe unrelated functions.

3. Final Answer:

   To remember user data across multiple pages -> Option A

4. Quick Check:

   Sessions remember user data = B [OK]

Hint: Sessions remember data across pages, not store on user PC [OK]

Common Mistakes:

• Confusing sessions with cookies stored on user device
• Thinking sessions speed up page loading
• Believing sessions create database connections

2. Which PHP function must be called before using $_SESSION variables?

easy

A. session_end()

B. session_open()

C. session_init()

D. session_start()

Solution

1. Step 1: Identify the correct function to start a session

   The function session_start() initializes a session or resumes the current one.

2. Step 2: Check other options

   Functions like session_end(), session_open(), and session_init() do not exist in PHP.

3. Final Answer:

   session_start() -> Option D

4. Quick Check:

   Start session with session_start() = C [OK]

Hint: Always call session_start() before accessing $_SESSION [OK]

Common Mistakes:

• Using non-existent functions like session_end()
• Forgetting to call session_start() before $_SESSION
• Calling session_start() after outputting HTML

3. What will be the output of this PHP code?

<?php
session_start();
$_SESSION['count'] = 1;
$_SESSION['count']++;
echo $_SESSION['count'];
?>

medium

A. 2

B. Error

C. 0

D. 1

Solution

1. Step 1: Analyze session variable assignment and increment

   The code sets $_SESSION['count'] to 1, then increments it by 1, making it 2.

2. Step 2: Output the session variable

   The echo statement prints the current value, which is 2.

3. Final Answer:

   2 -> Option A

4. Quick Check:

   1 incremented by 1 = 2 [OK]

Hint: Increment after setting 1 results in 2 [OK]

Common Mistakes:

• Assuming the value stays 1 without increment
• Thinking session variables reset each time
• Expecting an error without session_start()

4. Identify the error in this PHP code snippet:

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

medium

A. Cannot assign string to $_SESSION

B. Incorrect variable name, should be $SESSION

C. Missing session_start() before using $_SESSION

D. Echo statement syntax is wrong

Solution

1. Step 1: Check session initialization

   The code uses $_SESSION without calling session_start() first, which is required.

2. Step 2: Verify other options

   Variable name $_SESSION is correct, strings can be assigned, and echo syntax is valid.

3. Final Answer:

   Missing session_start() before using $_SESSION -> Option C

4. Quick Check:

   session_start() needed before $_SESSION = A [OK]

Hint: Always call session_start() before $_SESSION usage [OK]

Common Mistakes:

• Forgetting session_start() causes undefined $_SESSION
• Confusing $_SESSION with $SESSION
• Thinking echo syntax is incorrect

5. You want to count how many times a user visits a page during a session. Which code snippet correctly implements this?

hard

A. <?php
$_SESSION['visits'] = 1;
echo $_SESSION['visits'];
?>

B. <?php
session_start();
if (!isset($_SESSION['visits'])) {
$_SESSION['visits'] = 1;
} else {
$_SESSION['visits']++;
}
echo "Visits: " . $_SESSION['visits'];
?>

C. <?php
session_start();
$_SESSION['visits']++;
echo $_SESSION['visits'];
?>

D. <?php
session_start();
$_SESSION['visits'] = 0;
echo $_SESSION['visits'];
?>

Solution

1. Step 1: Check session initialization and visit count logic

   <?php session_start(); if (!isset($_SESSION['visits'])) { $_SESSION['visits'] = 1; } else { $_SESSION['visits']++; } echo "Visits: " . $_SESSION['visits']; ?> starts the session, checks if 'visits' exists, initializes or increments it properly.

2. Step 2: Review other options for errors

   <?php $_SESSION['visits'] = 1; echo $_SESSION['visits']; ?> lacks session_start(), <?php session_start(); $_SESSION['visits']++; echo $_SESSION['visits']; ?> increments without initialization, <?php session_start(); $_SESSION['visits'] = 0; echo $_SESSION['visits']; ?> resets visits to 0 every time.

3. Final Answer:

   The code snippet that initializes visits=1 if not set, else increments -> Option B

4. Quick Check:

   Initialize then increment visits with session_start() = A [OK]

Hint: Initialize visits if not set, then increment [OK]

Common Mistakes:

• Not calling session_start() before $_SESSION
• Incrementing without checking if variable exists
• Resetting counter on every page load