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

Concept Flow - While loop execution model

Initialize variable

↓

Check condition

|Yes↓

Execute loop body

↓

Update variable

Back to Check condition↓

Exit loop

↩Back to Check condition

The while loop starts by checking a condition. If true, it runs the loop body, updates variables, then checks the condition again. It stops when the condition is false.

Execution Sample

PHP

<?php
$i = 1;
while ($i <= 3) {
    echo $i . " ";
    $i++;
}
?>

This code prints numbers 1 to 3 using a while loop that runs while $i is less than or equal to 3.

Execution Table

Step	Variable $i	Condition ($i <= 3)	Action	Output
1	1	True	Print 1, increment $i to 2	1
2	2	True	Print 2, increment $i to 3	2
3	3	True	Print 3, increment $i to 4	3
4	4	False	Exit loop

💡 $i is 4, condition 4 <= 3 is False, loop stops

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
$i	1	2	3	4	4

Key Moments - 2 Insights

Why does the loop stop when $i is 4?

Because the condition $i <= 3 becomes false at step 4, so the loop exits as shown in the execution_table row 4.

When is the variable $i incremented in the loop?

The variable $i is incremented after printing, inside the loop body, as seen in the 'Action' column of the execution_table rows 1 to 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of $i at Step 3?

A 2

B 4

C 3

D 1

💡 Hint

Check the 'Variable $i' column at Step 3 in the execution_table.

At which step does the condition become false and the loop stops?

A Step 4

B Step 3

C Step 2

D Step 1

💡 Hint

Look at the 'Condition' column in the execution_table to find when it is false.

If we start $i at 0 instead of 1, how many times will the loop run?

A 3 times

B 4 times

C 2 times

D 1 time

💡 Hint

Refer to variable_tracker and consider the condition $i <= 3 starting from 0.

Concept Snapshot

while (condition) {
  // code to run
  // update variables
}

- Checks condition before each loop
- Runs loop body only if condition is true
- Stops when condition is false
- Update variables inside loop to avoid infinite loop

Full Transcript

This example shows how a while loop works in PHP. We start with a variable $i set to 1. The loop checks if $i is less than or equal to 3. If yes, it prints $i and then increases $i by 1. This repeats until $i becomes 4, when the condition fails and the loop stops. The execution table tracks each step, showing variable values, condition results, actions, and output. The variable tracker shows how $i changes after each step. Key moments explain why the loop stops and when $i is incremented. The quiz asks about variable values and loop behavior based on the tables. The snapshot summarizes the while loop syntax and rules.

Practice

1.

What does a while loop do in PHP?

easy

A. Runs code a fixed number of times

B. Repeats code as long as a condition is true

C. Executes code only once

D. Stops the program immediately

Solution

1. Step 1: Understand the while loop purpose

   A while loop repeats its code block while the condition remains true.

2. Step 2: Compare options with definition

   Only Repeats code as long as a condition is true matches this behavior exactly.

3. Final Answer:

   Repeats code as long as a condition is true -> Option B

4. Quick Check:

   while loop = repeat while true [OK]

Hint: Remember: while loops run only if condition is true [OK]

Common Mistakes:

• Thinking while loops run a fixed number of times
• Confusing while with if statement
• Assuming while loops always run at least once

2.

Which of the following is the correct syntax for a while loop in PHP?

?

easy

A. while ($i < 5); { echo $i; $i++; }

B. while $i < 5 { echo $i; $i++; }

C. while ($i < 5) { echo $i; $i++; }

D. while ($i < 5) echo $i; $i++;

Solution

1. Step 1: Check correct while loop syntax

   In PHP, the condition must be in parentheses and the block in braces.

2. Step 2: Identify syntax errors in options

   while ($i < 5) { echo $i; $i++; } uses correct parentheses and braces. Others miss braces, have semicolon errors, or missing parentheses.

3. Final Answer:

   while ($i < 5) { echo $i; $i++; } -> Option C

4. Quick Check:

   while (condition) { code } [OK]

Hint: Use parentheses for condition and braces for code block [OK]

Common Mistakes:

• Omitting parentheses around condition
• Adding semicolon after while condition
• Not using braces for multiple statements

3.

What will be the output of this PHP code?

$i = 1;
while ($i <= 3) {
    echo $i . ' ';
    $i++;
}

medium

A. 1 2 3

B. 1 2 3 4

C. 123

D. 2 3 4

Solution

1. Step 1: Trace the loop iterations

   Variable $i starts at 1 and increments by 1 each loop until $i > 3.

2. Step 2: Determine output each iteration

   Outputs '1 ', then '2 ', then '3 '. Loop stops after $i becomes 4.

3. Final Answer:

   1 2 3 -> Option A

4. Quick Check:

   Prints 1 to 3 with spaces [OK]

Hint: Count from start to condition limit inclusive [OK]

Common Mistakes:

• Including extra number after loop ends
• Missing spaces in output
• Confusing <= with < in condition

4.

Find the error in this PHP while loop code:

$i = 0;
while ($i < 3)
    echo $i;
    $i++;

medium

A. Condition syntax is wrong

B. echo statement is invalid

C. Variable $i is not initialized

D. Missing braces causes $i++ to run outside loop

Solution

1. Step 1: Analyze loop structure without braces

   Without braces, only the first statement after while runs in loop.

2. Step 2: Identify effect on $i++

   $i++ runs after loop ends, causing infinite loop because $i never increments inside loop.

3. Final Answer:

   Missing braces causes $i++ to run outside loop -> Option D

4. Quick Check:

   Braces needed to include all statements in loop [OK]

Hint: Use braces to group multiple statements in while loop [OK]

Common Mistakes:

• Assuming indentation controls loop body
• Forgetting to increment loop variable inside loop
• Thinking condition syntax is incorrect

5.

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

?

hard

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

B. $i = 1;
while ($i <= 10) {
if ($i % 2 == 1) echo $i . ' ';
$i++;
}

C. $i = 2;
while ($i < 10) {
echo $i . ' ';
$i += 2;
}

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

Solution

1. Step 1: Identify correct start and condition

   Start at 2 and include 10, so condition is $i <= 10.

2. Step 2: Check increment and output

   Increment by 2 to get even numbers only, and echo with space.

3. Step 3: Compare options

   $i = 2; while ($i <= 10) { echo $i . ' '; $i += 2; } matches all criteria exactly. $i = 1; while ($i <= 10) { if ($i % 2 == 1) echo $i . ' '; $i++; } prints odd numbers: 1 3 5 7 9. $i = 2; while ($i < 10) { echo $i . ' '; $i += 2; } misses 10. $i = 0; while ($i <= 10) { echo $i . ' '; $i += 2; } starts at 0, which is not requested.

4. Final Answer:

   $i = 2; while ($i <= 10) { echo $i . ' '; $i += 2; } -> Option A

5. Quick Check:

   Start=2, end=10, step=2 [OK]

Hint: Start at 2, increment by 2, stop at 10 inclusive [OK]

Common Mistakes:

• Starting at 0 or 1 instead of 2
• Using < instead of <= to exclude 10
• Not incrementing by 2 to get even numbers