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.