For loop execution model in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are the three main parts of a for loop in PHP?

The three main parts are:
1. Initialization (runs once at the start)
2. Condition (checked before each loop iteration)
3. Increment/Decrement (runs after each iteration)

beginner

Explain the order of execution in a PHP for loop.

First, the initialization runs once.
Then, the condition is checked.
If true, the loop body runs.
After the body, the increment/decrement runs.
This repeats until the condition is false.

beginner

What happens if the condition in a for loop is false at the start?

The loop body does not run at all.
The program skips the loop and continues after it.

intermediate

Can the increment part of a for loop be empty in PHP? What does that mean?

Yes, it can be empty.
This means no automatic change happens after each iteration.
You must manually change the loop variable inside the loop body to avoid infinite loops.

beginner

Show a simple PHP for loop that prints numbers 1 to 5.

Example:

<?php
for ($i = 1; $i <= 5; $i++) {
    echo $i . "\n";
}
?>

This prints numbers 1, 2, 3, 4, 5 each on a new line.

In a PHP for loop, which part runs only once at the start?

A Condition

B Initialization

C Increment

D Loop body

Initialization runs once before the loop starts to set up the loop variable.

What happens if the condition in a PHP for loop is false initially?

A The loop body does not run

B The loop runs infinitely

C The increment runs once

D The loop runs once

If the condition is false at the start, the loop body does not execute at all.

Which part of the for loop runs after each iteration?

A Increment

B Condition

C Initialization

D Loop body

The increment part runs after each loop iteration to update the loop variable.

Is it possible to have an empty increment section in a PHP for loop?

A Yes, and the loop will run infinitely

B No, it is required

C Yes, but you must update the variable inside the loop

D No, it causes a syntax error

You can leave increment empty but must update the loop variable inside the loop to avoid infinite loops.

What is the correct syntax for a PHP for loop that counts from 0 to 4?

A for ($i = 5; $i > 0; $i--)

B for ($i = 1; $i <= 5; $i++)

C for ($i = 0; $i <= 5; $i--)

D for ($i = 0; $i < 5; $i++)

Option D correctly counts from 0 up to 4 by incrementing $i until it is less than 5.

Describe the execution flow of a PHP for loop from start to finish.

Think about what happens before, during, and after each loop cycle.

Explain what happens if you forget to update the loop variable inside a for loop with an empty increment section.

Consider what keeps the loop going or stopping.

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