Script execution and memory reset in PHP - Step-by-Step Execution

Concept Flow - Script execution and memory reset

Start script execution

↓

Execute code line by line

↓

Variables stored in memory

↓

End of script reached

↓

Memory cleared, variables reset

↓

Script execution ends

PHP runs the script from top to bottom, storing variables in memory during execution. When the script ends, all memory is cleared and variables reset.

Execution Sample

PHP

<?php
$a = 5;
echo $a;
?>

This script sets a variable $a to 5, prints it, then ends, clearing memory.

Execution Table

Step	Action	Variable State	Output
1	Start script execution	No variables
2	Assign $a = 5	$a = 5
3	Print $a	$a = 5	5
4	End of script	$a = 5 (about to clear)
5	Memory cleared	No variables

💡 Script ends, PHP clears all variables from memory

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5
$a	undefined	5	5	undefined

Key Moments - 2 Insights

Why does $a become undefined after the script ends?

Because PHP clears all variables from memory when the script finishes, as shown in execution_table step 5.

Does the value of $a persist between different script runs?

No, each script run starts fresh with no variables, as shown in execution_table step 1 and 5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of $a after step 2?

A undefined

B 0

C 5

D null

💡 Hint

Check the 'Variable State' column at step 2 in execution_table

At which step does the script output the value 5?

A Step 1

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Output' column in execution_table

If the script had another line after step 3 that changed $a to 10, what would be $a's value at step 4?

A 10

B 5

C undefined

D null

💡 Hint

Variable changes persist until script ends, see variable_tracker pattern

Concept Snapshot

PHP script runs top to bottom.
Variables exist only during execution.
At script end, all variables are cleared.
Each run starts with fresh memory.
Output happens during execution only.

Full Transcript

This visual trace shows how a PHP script executes line by line. First, the script starts with no variables in memory. Then, the variable $a is assigned the value 5. Next, the script prints the value of $a, which outputs 5. After the script reaches the end, PHP clears all variables from memory, resetting $a to undefined. This means each time the script runs, it starts fresh with no leftover data from before. Understanding this helps beginners know why variables do not keep their values between script runs.

Practice

1. What happens to variables in a PHP script after the script finishes running?

easy

A. Variables are saved permanently in the script file

B. Variables keep their values until the server restarts

C. All variables and memory are reset and cleared

D. Variables are shared with other scripts automatically

Solution

1. Step 1: Understand PHP script execution

   PHP runs scripts from top to bottom each time the page loads.

2. Step 2: Memory and variable reset after execution

   After the script finishes, all variables and memory used are cleared to keep the environment clean.

3. Final Answer:

   All variables and memory are reset and cleared -> Option C

4. Quick Check:

   PHP resets variables after script ends = C [OK]

Hint: Remember PHP resets memory after each script run [OK]

Common Mistakes:

• Thinking variables keep values between page loads
• Assuming variables save to the script file
• Believing variables are shared across scripts automatically

2. Which of the following is the correct way to start a PHP script that resets variables each time it runs?

easy

A.

B.

C.

D. // PHP code here

Solution

1. Step 1: Identify correct PHP tags

   PHP scripts start with to run properly.

2. Step 2: Understand script resets

   Each time the script runs, variables reset automatically without extra commands.

3. Final Answer:

   -> Option A

4. Quick Check:

   Correct PHP tags = A [OK]

Hint: Use tags to start PHP scripts [OK]

Common Mistakes:

• Using <script> tags which are for JavaScript
• Trying to call reset() function which doesn't reset variables
• Using incorrect <php> tags

3. Consider this PHP code run twice in a row (two separate page loads):

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

What will be the output each time the script runs?

medium

A. 1 then 1

B. 0 then 1

C. 1 then 2

D. 0 then 0

Solution

1. Step 1: Analyze variable initialization

   Each time the script runs, $counter is set to 0 at the start.

2. Step 2: Increment and output

   $counter++ increases $counter to 1, then echo prints 1 each time.

3. Final Answer:

   1 then 1 -> Option A

4. Quick Check:

   Variables reset each run, so output is 1 each time [OK]

Hint: Variables reset each run, so counter restarts at 0 [OK]

Common Mistakes:

• Assuming $counter keeps incrementing across runs
• Confusing pre-increment and post-increment
• Thinking echo prints 0 first

4. You wrote this PHP code:

<?php
$sum = 10;
$sum += 5;
?>

But after refreshing the page, $sum is always 15. You want $sum to keep increasing by 5 each time the page loads. What is the problem?

medium

A. You need to use a loop to increase $sum

B. The += operator does not add values correctly

C. You forgot to declare $sum as global

D. PHP resets variables after each script run, so $sum starts at 10 every time

Solution

1. Step 1: Understand variable reset behavior

   PHP clears all variables after each script finishes, so $sum resets to 10 every time.

2. Step 2: Why $sum does not keep increasing

   Because $sum is reinitialized to 10 on each page load, adding 5 results in 15 always.

3. Final Answer:

   PHP resets variables after each script run, so $sum starts at 10 every time -> Option D

4. Quick Check:

   Variables reset each run = D [OK]

Hint: Variables reset each run; use sessions or files to keep values [OK]

Common Mistakes:

• Thinking += operator is faulty
• Assuming global declaration keeps value between runs
• Believing loops affect variable persistence

5. You want to count how many times a PHP page has been loaded by visitors. Since PHP resets variables each run, which method should you use to keep the count across page loads?

hard

A. Declare the count variable as global inside the script

B. Store the count in a session or a file/database to persist data

C. Use a loop to increment the count variable inside the script

D. Reset the count variable to zero at the start of the script

Solution

1. Step 1: Recognize PHP variable reset limitation

   PHP variables reset after each script run, so count cannot persist in normal variables.

2. Step 2: Use persistent storage

   Sessions, files, or databases store data between page loads to keep count.

3. Final Answer:

   Store the count in a session or a file/database to persist data -> Option B

4. Quick Check:

   Persistent storage needed for count across runs = B [OK]

Hint: Use sessions or files to save data between page loads [OK]

Common Mistakes:

• Thinking global variables keep values between runs
• Trying to use loops for persistence
• Resetting variables each time defeats counting