For loop execution model in PHP - Step-by-Step Execution

Concept Flow - For loop execution model

Initialize i=0

↓

Check: i < 3?

No→EXIT

Yes↓

Execute body: print i

↓

Update: i = i+1

↩Back to Check

The loop starts by setting i to 0, then checks if i is less than 3. If yes, it runs the loop body, then increases i by 1 and repeats. If no, it stops.

Execution Sample

PHP

<?php
for ($i = 0; $i < 3; $i++) {
    echo $i . "\n";
}
?>

This PHP code prints numbers 0, 1, and 2 each on a new line using a for loop.

Execution Table

Iteration	i before condition	Condition (i < 3)	Body executed	Output	i after update
1	0	True	Yes	0	1
2	1	True	Yes	1	2
3	2	True	Yes	2	3
4	3	False	No

💡 i reaches 3, condition 3 < 3 is False, loop stops

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
i	0	1	2	3	3

Key Moments - 3 Insights

Why does the loop stop when i equals 3?

Because the condition i < 3 becomes False at iteration 4 (see execution_table row 4), so the loop does not run the body again.

When is the variable i updated in the loop?

i is updated after the loop body executes each time (see execution_table column 'i after update'), before the next condition check.

What is printed during each iteration?

The current value of i before it is incremented is printed (see execution_table 'Output' column for iterations 1 to 3).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of i before the condition check in iteration 3?

A 3

B 2

C 1

D 0

💡 Hint

Check the 'i before condition' column in row 3 of the execution_table.

At which iteration does the loop condition become false?

A Iteration 4

B Iteration 1

C Iteration 3

D Iteration 2

💡 Hint

Look at the 'Condition (i < 3)' column and find where it is False.

If the loop condition changed to i < 2, how many times would the loop body execute?

A 3 times

B 1 time

C 2 times

D 0 times

💡 Hint

Refer to the variable_tracker and execution_table to see how condition affects iterations.

Concept Snapshot

PHP for loop syntax:
for (initialization; condition; update) {
  // code to run
}

Loop runs while condition is true.
Initialization runs once at start.
Update runs after each loop body execution.
Loop stops when condition is false.

Full Transcript

This visual execution model shows how a PHP for loop works step-by-step. The loop starts by setting i to 0. Then it checks if i is less than 3. If yes, it runs the loop body which prints i, then increases i by 1. This repeats until i reaches 3, when the condition becomes false and the loop stops. The execution table tracks each iteration's variable values, condition results, and outputs. The variable tracker shows how i changes from 0 to 3. Key moments clarify why the loop stops at 3 and when i updates. The quiz tests understanding of variable values and loop behavior. This helps beginners see exactly how the for loop runs in PHP.

Practice

1. What is the role of the condition part in a PHP for loop?

easy

A. It decides if the loop should continue running or stop.

B. It sets the starting value of the loop variable.

C. It changes the loop variable after each iteration.

D. It runs only after the loop finishes.

Solution

1. Step 1: Understand the for loop parts

   A PHP for loop has three parts: initialization, condition, and increment.

2. Step 2: Identify the condition's role

   The condition is checked before each loop run to decide if the loop continues or stops.

3. Final Answer:

   It decides if the loop should continue running or stop. -> Option A

4. Quick Check:

   Condition controls loop continuation = It decides if the loop should continue running or stop. [OK]

Hint: Condition controls loop stop or continue [OK]

Common Mistakes:

• Confusing initialization with condition
• Thinking increment runs before loop starts
• Believing condition runs after loop ends

2. Which of the following is the correct syntax for a PHP for loop that counts from 0 to 4?

easy

A. for $i = 0; $i <= 4; $i++ {}

B. for ($i = 0; $i < 4; $i--) {}

C. for ($i = 0; $i <= 4; ++i) {}

D. for ($i = 0; $i <= 4; $i++) {}

Solution

1. Step 1: Check for loop syntax rules

   PHP for loops require parentheses around initialization, condition, and increment, separated by semicolons.

2. Step 2: Validate each option

   for ($i = 0; $i <= 4; $i++) {} uses correct syntax with $i starting at 0, condition $i <= 4, and increment $i++. Others have syntax errors or wrong increments.

3. Final Answer:

   for ($i = 0; $i <= 4; $i++) {} -> Option D

4. Quick Check:

   Correct for loop syntax = for ($i = 0; $i <= 4; $i++) {} [OK]

Hint: Use parentheses and semicolons correctly in for loops [OK]

Common Mistakes:

• Missing parentheses around for loop parts
• Using ++i without $ sign
• Wrong increment direction (i-- instead of i++)

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

$sum = 0;
for ($i = 1; $i <= 3; $i++) {
    $sum += $i;
}
echo $sum;

medium

A. 6

B. 123

C. 3

D. 0

Solution

1. Step 1: Trace the loop iterations

   The loop runs with $i = 1, 2, 3. Each time, $sum adds $i: 0+1=1, 1+2=3, 3+3=6.

2. Step 2: Calculate final sum

   After the loop, $sum is 6, which is printed by echo.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Sum 1+2+3 = 6 [OK]

Hint: Add loop variable each iteration to sum [OK]

Common Mistakes:

• Concatenating numbers instead of adding
• Starting loop at 0 instead of 1
• Using wrong loop condition

4. Identify the error in this PHP for loop:

for ($i = 0; $i < 5; $i) {
    echo $i;
}

medium

A. Condition should be $i <= 5.

B. Missing increment operation in the loop header.

C. Initialization is incorrect.

D. Echo statement syntax is wrong.

Solution

1. Step 1: Examine the for loop header

   The loop header has initialization ($i = 0), condition ($i < 5), but increment is just $i without ++ or --.

2. Step 2: Identify the missing increment

   Without increment like $i++, the loop variable never changes, causing an infinite loop or no progress.

3. Final Answer:

   Missing increment operation in the loop header. -> Option B

4. Quick Check:

   Increment missing = Missing increment operation in the loop header. [OK]

Hint: Ensure increment changes loop variable each time [OK]

Common Mistakes:

• Forgetting ++ or -- in increment
• Changing condition to wrong comparison
• Misplacing semicolons in loop header

5. You want to print all even numbers from 2 to 10 using a PHP for loop. Which code correctly does this?

hard

A. for ($i = 2; $i < 10; $i += 2) { echo $i . ' '; }

B. for ($i = 0; $i <= 10; $i += 2) { echo $i . ' '; }

C. for ($i = 1; $i <= 10; $i++) { if ($i % 2 == 0) echo $i . ' '; }

D. for ($i = 2; $i < 10; $i++) { if ($i % 2 == 0) echo $i . ' '; }

Solution

1. Step 1: Understand the goal

   We want to print even numbers from 2 to 10 inclusive.

2. Step 2: Check each option

   for ($i = 2; $i < 10; $i += 2) { echo $i . ' '; } prints 2 4 6 8, missing 10. for ($i = 1; $i <= 10; $i++) { if ($i % 2 == 0) echo $i . ' '; } loops 1 to 10 and prints only even numbers 2 4 6 8 10 correctly. for ($i = 2; $i < 10; $i++) { if ($i % 2 == 0) echo $i . ' '; } stops at 9, missing 10. for ($i = 0; $i <= 10; $i += 2) { echo $i . ' '; } starts at 0, which is not in the range 2 to 10.

3. Step 3: Choose the best option

   for ($i = 1; $i <= 10; $i++) { if ($i % 2 == 0) echo $i . ' '; } is correct and flexible, printing even numbers by checking inside the loop.

4. Final Answer:

   for ($i = 1; $i <= 10; $i++) { if ($i % 2 == 0) echo $i . ' '; } -> Option C

5. Quick Check:

   Loop 1-10, print if even = for ($i = 1; $i <= 10; $i++) { if ($i % 2 == 0) echo $i . ' '; } [OK]

Hint: Use modulo % to check even numbers inside loop [OK]

Common Mistakes:

• Stopping loop before 10
• Starting loop at 0 when not needed
• Forgetting to check even condition