Starting and using sessions in PHP - Step-by-Step Execution

Concept Flow - Starting and using sessions

Start PHP script

↓

Call session_start()

↓

Check if session exists

↓

Create or resume session

↓

Store or retrieve data in $_SESSION

↓

Use session data in script

↓

End script

The script starts, calls session_start() to create or resume a session, then stores or retrieves data in the $_SESSION superglobal for use during the script.

Execution Sample

PHP

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

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

Execution Table

Step	Action	Session Status	$_SESSION Content	Output
1	Call session_start()	Session started or resumed	{} (empty or existing)
2	Assign 'Alice' to $_SESSION['user']	Session active	{"user": "Alice"}
3	Echo $_SESSION['user']	Session active	{"user": "Alice"}	Alice
4	Script ends	Session data saved	{"user": "Alice"}

💡 Script ends after outputting session data.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
$_SESSION	{}	{"user": "Alice"}	{"user": "Alice"}	{"user": "Alice"}

Key Moments - 3 Insights

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

session_start() must be called before accessing or modifying $_SESSION; it creates or resumes the session. See step 1 in execution_table.

What happens if we try to use $_SESSION without session_start()?

Without session_start(), $_SESSION is not initialized, so data won't be saved or retrieved properly. The session won't exist (step 1).

How does PHP remember session data between page loads?

PHP uses a session cookie to link the browser to stored session data on the server, managed after session_start() (step 1).

Visual Quiz - 3 Questions

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

A null

B {"user": "Alice"}

C {}

D {"user": "Bob"}

💡 Hint

Check the '$_SESSION Content' column at step 2 in execution_table.

At which step does the script output 'Alice'?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Output' column in execution_table.

If session_start() is removed, what happens to $_SESSION?

A It still stores data normally

B It causes a syntax error

C It remains empty and no session data is saved

D It automatically starts the session

💡 Hint

Refer to key_moments about the importance of session_start() and step 1 in execution_table.

Concept Snapshot

PHP sessions store data across pages.
Call session_start() first to create or resume a session.
Use $_SESSION superglobal to save or get data.
Session data persists via a cookie linking browser and server.
Always call session_start() before accessing $_SESSION.

Full Transcript

This example shows how PHP sessions work step-by-step. First, the script calls session_start() to create or resume a session. Then it stores the string 'Alice' in the $_SESSION array under the key 'user'. Next, it prints the stored value. The session data is saved on the server and linked to the browser by a cookie. Without calling session_start(), the $_SESSION array is not available and data won't persist. This trace helps beginners see how session data is created, stored, and used in PHP scripts.

Practice

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

easy

A. To begin a session and enable storing data across pages

B. To end a session and clear all session data

C. To connect to a database for user authentication

D. To send cookies to the user's browser

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: Recognize session purpose

   Sessions allow data to be saved and accessed across multiple pages for the same user, such as login status or preferences.

3. Final Answer:

   To begin a session and enable storing data across pages -> Option A

4. Quick Check:

   session_start() starts session = D [OK]

Hint: Remember: session_start() must run before any output [OK]

Common Mistakes:

• Thinking session_start() ends a session
• Confusing sessions with database connections
• Calling session_start() after HTML output

2. Which of the following is the correct way to start a session in PHP?

easy

A. session.begin();

B. start_session();

C. session_start();

D. begin_session();

Solution

1. Step 1: Recall PHP session function syntax

   The correct function to start a session is session_start() with an underscore and parentheses.

2. Step 2: Check other options for syntax errors

   Options B, C, and D are not valid PHP functions and will cause errors.

3. Final Answer:

   session_start(); -> Option C

4. Quick Check:

   Correct PHP session start syntax = A [OK]

Hint: Use exact function name: session_start() [OK]

Common Mistakes:

• Using wrong function names like start_session()
• Missing parentheses after session_start
• Using camelCase instead of underscore

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

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

medium

A. user

B. Alice

C. Array

D. Undefined index error

Solution

1. Step 1: Start session and assign value

   The code starts a session and sets the session variable 'user' to 'Alice'.

2. Step 2: Output the session variable

   Using echo $_SESSION['user']; prints the value stored, which is 'Alice'.

3. Final Answer:

   Alice -> Option B

4. Quick Check:

   $_SESSION['user'] = 'Alice' outputs Alice [OK]

Hint: Session variables store values like arrays [OK]

Common Mistakes:

• Expecting 'user' instead of its value
• Forgetting to call session_start() first
• Confusing session variables with regular variables

4. Identify the error in this PHP code snippet:

<?php
$_SESSION['count'] = 1;
echo $_SESSION['count'];
?>

medium

A. Cannot echo session variables

B. Incorrect variable name $_SESSION

C. Session variables must be integers only

D. Missing session_start() before using $_SESSION

Solution

1. Step 1: Check session initialization

   The code uses $_SESSION but never calls session_start() to begin the session.

2. Step 2: Understand session usage rules

   Without calling session_start(), PHP does not know which session to use, causing errors or empty values.

3. Final Answer:

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

4. Quick Check:

   Always call session_start() before $_SESSION [OK]

Hint: Always start session before accessing $_SESSION [OK]

Common Mistakes:

• Using $_SESSION without session_start()
• Assuming $_SESSION works like normal variables
• Trying to echo $_SESSION without initialization

5. You want to count how many times a user visits a page using sessions. Which code correctly increments the visit count each time the page loads?

<?php
// Fill in the missing part

if (isset($_SESSION['visits'])) {
    $_SESSION['visits']++;
} else {
    $_SESSION['visits'] = 1;
}
echo "Visit number: " . $_SESSION['visits'];
?>

hard

A. session_start();

B. session_destroy();

C. session_unset();

D. session_write_close();

Solution

1. Step 1: Recognize session must be started

   To use $_SESSION variables, the session must be started with session_start(); at the top.

2. Step 2: Understand the code logic

   The code checks if 'visits' exists in the session, increments it if yes, or sets it to 1 if no, then prints the count.

3. Final Answer:

   session_start(); -> Option A

4. Quick Check:

   Start session before using $_SESSION to track visits [OK]

Hint: Always call session_start() before session variables [OK]

Common Mistakes:

• Forgetting session_start() causes no session data
• Using session_destroy() resets visits
• Confusing session_unset() with starting session