Why variables do not persist between requests in PHP - Performance Analysis

Time Complexity: Why variables do not persist between requests

O(1)

Understanding Time Complexity

When a PHP script runs, it starts fresh each time a user visits a page.

We want to understand why variables don't keep their values between these visits.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

<?php
$counter = 0;
$counter++;
echo $counter;
?>
    

This code sets a variable, increases it, and shows it each time the page loads.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Assigning and incrementing a variable once per request.
• How many times: Exactly once each time the script runs (each page load).

How Execution Grows With Input

Each time the page loads, the script runs from the start, so the variable resets.

Input Size (n)	Approx. Operations
1 page load	1 increment operation
10 page loads	10 separate increments, each starting fresh
100 page loads	100 separate increments, no memory of past values

Pattern observation: The variable does not remember past increments because each run is independent.

Final Time Complexity

Time Complexity: O(1)

This means the script does a fixed amount of work each time, no matter how many times it runs.

Common Mistake

[X] Wrong: "The variable will keep increasing every time the page reloads."

[OK] Correct: Each page load starts a new script with fresh variables, so previous values are lost.

Interview Connect

Understanding this helps you explain how web servers handle requests and why you need special ways to save data between visits.

Self-Check

"What if we used sessions or files to store the variable? How would the time complexity change?"

Practice

1. Why do PHP variables not keep their values between different page requests?

easy

A. Because PHP variables are saved only in cookies

B. Because PHP automatically deletes variables after 5 seconds

C. Because variables are stored only in the browser cache

D. Because each request starts a new script execution with fresh variables

Solution

1. Step 1: Understand PHP script execution

   Each time a PHP page loads, the server runs the script from the start with new variables.

2. Step 2: Recognize variable lifetime

   Variables exist only during that single script run and are lost after the page finishes loading.

3. Final Answer:

   Because each request starts a new script execution with fresh variables -> Option D

4. Quick Check:

   PHP variables reset each request = D [OK]

Hint: Remember: PHP runs fresh each page load, no memory kept [OK]

Common Mistakes:

• Thinking variables persist like JavaScript variables in browser
• Confusing PHP variables with cookies or sessions
• Assuming PHP stores variables on client side

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

easy

A. session_start();

B. start_session();

C. begin_session();

D. init_session();

Solution

1. Step 1: Recall PHP session function

   The correct function to start a session is session_start();.

2. Step 2: Verify syntax correctness

   Other options are not valid PHP functions and will cause errors.

3. Final Answer:

   session_start(); -> Option A

4. Quick Check:

   Use session_start() to begin sessions [OK]

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

Common Mistakes:

• Using incorrect function names like start_session()
• Forgetting parentheses after function name
• Not calling session_start() before output

3. What will be the output of this PHP code on the second page load?

<?php
$counter = 0;
$counter++;
echo $counter;
?>

medium

A. 1

B. 2

C. 0

D. Error

Solution

1. Step 1: Analyze variable initialization

   Each page load sets $counter = 0;, so it resets every time.

2. Step 2: Increment and output

   Then $counter++ makes it 1, and echo prints 1.

3. Final Answer:

   1 -> Option A

4. Quick Check:

   Variable resets each load, so output is 1 [OK]

Hint: Variables reset each request unless stored externally [OK]

Common Mistakes:

• Assuming $counter keeps incrementing across requests
• Thinking output will be 2 or higher
• Expecting an error due to variable usage

4. This PHP code tries to keep a count between requests but always shows 1. What is the problem?

<?php
$counter = 0;
$counter++;
echo $counter;
?>

medium

A. The increment operator ++ is used incorrectly

B. The echo statement is missing a semicolon

C. Variables reset each request; no session or storage used

D. The variable $counter is not declared

Solution

1. Step 1: Identify variable reset issue

   The variable $counter is set to 0 on every request, so it never remembers previous value.

2. Step 2: Understand missing persistence method

   Without sessions or external storage, variables cannot keep values between requests.

3. Final Answer:

   Variables reset each request; no session or storage used -> Option C

4. Quick Check:

   Variables reset without session = B [OK]

Hint: Use sessions to keep variables between requests [OK]

Common Mistakes:

• Blaming increment operator for reset
• Thinking echo needs semicolon to work
• Assuming variable declaration is missing

5. You want to count how many times a user visits a page using PHP. Which approach correctly keeps the count between requests?

hard

A. Declare a global variable and increment it on each request

B. Use a session variable with session_start() and increment it

C. Use a local variable inside the script and increment it

D. Store the count in a variable inside the HTML code

Solution

1. Step 1: Understand variable lifetime

   Local and global PHP variables reset each request, so they can't keep count.

2. Step 2: Use sessions for persistence

   Sessions store data on the server between requests, so using session_start() and a session variable keeps count.

3. Final Answer:

   Use a session variable with session_start() and increment it -> Option B

4. Quick Check:

   Sessions keep data between requests = A [OK]

Hint: Use sessions to save data across page loads [OK]

Common Mistakes:

• Thinking global variables persist across requests
• Trying to store PHP variables in HTML
• Not starting session before using session variables