Session variables in PHP - Step-by-Step Execution

Concept Flow - Session variables

Start PHP script

↓

Call session_start()

↓

Check if session exists

↓

Set or read $_SESSION variables

↓

Use session data in script

↓

End script, session data saved

↓

Next request: session_start() resumes session

↓

Access $_SESSION variables again

The script starts a session, sets or reads session variables stored on the server, and these variables persist across page loads.

Execution Sample

PHP

<?php
session_start();
$_SESSION['count'] = ($_SESSION['count'] ?? 0) + 1;
echo "Count: " . $_SESSION['count'];
?>

This code starts a session, increases a counter stored in session, and prints it each time the page loads.

Execution Table

Step	Action	$_SESSION['count'] before	Operation	$_SESSION['count'] after	Output
1	Start session, no count set	undefined	Initialize count to 0 + 1	1	Count: 1
2	Reload page, session resumes	1	Increment count by 1	2	Count: 2
3	Reload page again	2	Increment count by 1	3	Count: 3
4	Reload page again	3	Increment count by 1	4	Count: 4

💡 Execution stops after output; session data saved for next request.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4
$_SESSION['count']	undefined	1	2	3	4

Key Moments - 3 Insights

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

session_start() tells PHP to load or create the session data so $_SESSION variables can be accessed or set, as shown in step 1 of the execution_table.

What happens if we don't check if $_SESSION['count'] exists before incrementing?

Using the null coalescing operator (??) in the code ensures we start from 0 if 'count' is not set, preventing errors or warnings, as seen in step 1 where count was undefined before initialization.

Why does the count keep increasing on each page reload?

Because the session data is saved on the server and linked to the user, each reload resumes the session and increments the stored count, shown in steps 2 to 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of $_SESSION['count'] after the second page reload?

A 3

B 1

C 2

D undefined

💡 Hint

Check the 'After 2' column in variable_tracker or step 2 in execution_table.

At which step does $_SESSION['count'] get initialized from undefined to 1?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the first row in execution_table where count changes from undefined.

If session_start() was missing, what would happen to $_SESSION['count']?

A It would cause an error or not persist between requests.

B It would reset to 0 every time but no error.

C It would still increment normally.

D It would double on each reload.

💡 Hint

Recall the key_moments explanation about session_start() necessity.

Concept Snapshot

Session variables store data on the server linked to a user.
Use session_start() at script start to access $_SESSION.
Set or read $_SESSION['key'] to save data across pages.
Data persists until session ends or is cleared.
Useful for login states, counters, and user info.

Full Transcript

This visual execution shows how PHP session variables work. The script begins by calling session_start() to begin or resume a session. Then it checks if a session variable 'count' exists. If not, it initializes it to zero and adds one. Each time the page reloads, the session resumes, and the count increases by one. The execution table traces each step, showing the value of $_SESSION['count'] before and after incrementing, and the output printed. The variable tracker summarizes how 'count' changes from undefined to 1, then 2, 3, and so on. Key moments clarify why session_start() is needed, how the null coalescing operator prevents errors, and why the count persists across reloads. The quiz tests understanding of these steps. The snapshot summarizes session variables as server-side storage linked to users, requiring session_start(), and useful for keeping data across pages.

Practice

1. What is the main purpose of using session_start() in PHP?

easy

A. To connect to a database

B. To send output to the browser

C. To begin a session and enable session variables

D. To include another PHP file

Solution

1. Step 1: Understand what session_start() does

   This function initializes a session or resumes the current one based on a session identifier passed via a cookie or URL.

2. Step 2: Identify the purpose of session variables

   Session variables store user data across multiple pages, but they only work if a session is started first.

3. Final Answer:

   To begin a session and enable session variables -> Option C

4. Quick Check:

   session_start() = start session [OK]

Hint: Remember: session_start() must be called before using $_SESSION [OK]

Common Mistakes:

• Thinking session_start() connects to a database
• Using session variables without calling session_start()
• Confusing session_start() with output functions

2. Which of the following is the correct way to set a session variable named user with the value "Alice"?

easy

A. $_SESSION['user'] = 'Alice';

B. session['user'] = 'Alice';

C. set_session('user', 'Alice');

D. session_start['user'] = 'Alice';

Solution

1. Step 1: Recall the syntax for setting session variables

   Session variables are stored in the global $_SESSION array, so you assign values like $_SESSION['key'] = value;.

2. Step 2: Check each option's syntax

   $_SESSION['user'] = 'Alice'; uses the correct $_SESSION superglobal with proper array syntax and assignment.

3. Final Answer:

   $_SESSION['user'] = 'Alice'; -> Option A

4. Quick Check:

   $_SESSION['key'] = value [OK]

Hint: Use $_SESSION['name'] = value to set session data [OK]

Common Mistakes:

• Using session without the $_SESSION superglobal
• Trying to call session_start as an array
• Using undefined functions like set_session()

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

session_start();
$_SESSION['count'] = 1;
$_SESSION['count']++;
echo $_SESSION['count'];

medium

A. 1

B. 0

C. Error

D. 2

Solution

1. Step 1: Analyze session variable initialization and increment

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

2. Step 2: Determine the final value of $_SESSION['count']

   After increment, the value becomes 2, which is then printed by echo.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   1 incremented by 1 = 2 [OK]

Hint: Increment session variable with ++ after setting it [OK]

Common Mistakes:

• Forgetting to call session_start() before using $_SESSION
• Assuming $_SESSION['count'] stays 1 after ++
• Expecting an error due to session usage

4. Identify the error in this PHP code snippet:

$_SESSION['user'] = 'Bob';
echo $_SESSION['user'];

medium

A. Missing session_start() before using $_SESSION

B. Incorrect array syntax for $_SESSION

C. Cannot assign string to $_SESSION

D. Echo statement syntax is wrong

Solution

1. Step 1: Check for session initialization

   The code uses $_SESSION but does not call session_start() first, which is required to access session variables.

2. Step 2: Verify syntax correctness

   The array syntax and echo statement are correct, and assigning a string to $_SESSION is valid.

3. Final Answer:

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

4. Quick Check:

   Always call session_start() before $_SESSION [OK]

Hint: Always start session with session_start() before $_SESSION use [OK]

Common Mistakes:

• Using $_SESSION without session_start()
• Thinking $_SESSION needs special syntax
• Assuming 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 using session variables?

hard

A. session_start();
$_SESSION['visits']++;
echo $_SESSION['visits'];

B. session_start();
if (!isset($_SESSION['visits'])) {
$_SESSION['visits'] = 1;
} else {
$_SESSION['visits']++;
}
echo $_SESSION['visits'];

C. session_start();
$_SESSION['visits'] = 1;
echo $_SESSION['visits'];

D. if (!isset($_SESSION['visits'])) {
$_SESSION['visits'] = 1;
} else {
$_SESSION['visits']++;
}
echo $_SESSION['visits'];

Solution

1. Step 1: Ensure session is started

   Calling session_start() is necessary to use session variables.

2. Step 2: Check visit count logic

   The code checks if $_SESSION['visits'] exists; if not, it sets it to 1. Otherwise, it increments the count. This correctly tracks visits.

3. Step 3: Verify output

   Finally, it echoes the current visit count, showing how many times the user visited during the session.

4. Final Answer:

   Code snippet with session_start() and visit count check -> Option B

5. Quick Check:

   Initialize then increment visits with session_start() [OK]

Hint: Always check isset() before incrementing session counters [OK]

Common Mistakes:

• Forgetting session_start() before using $_SESSION
• Not checking if session variable exists before increment
• Resetting visit count every page load