Nested loop execution in PHP - Step-by-Step Execution

Concept Flow - Nested loop execution

Start Outer Loop i=0

↓

Start Inner Loop j=0

↓

Execute Inner Loop Body

↓

Update j

↓

Check j < limit?

Yes→Repeat Inner Loop

No↓

Update i

↓

Check i < limit?

Yes→Repeat Outer Loop

No↓

End

The outer loop starts and for each iteration, the inner loop runs completely before the outer loop moves to the next iteration.

Execution Sample

PHP

<?php
for ($i = 0; $i < 2; $i++) {
    for ($j = 0; $j < 3; $j++) {
        echo "$i,$j\n";
    }
}
?>

This code runs two loops: the outer loop runs 2 times, and for each outer loop, the inner loop runs 3 times, printing pairs of i and j.

Execution Table

Step	i	j	Condition i<2	Condition j<3	Action	Output
1	0	0	True	True	Print '0,0'	0,0
2	0	1	True	True	Print '0,1'	0,1
3	0	2	True	True	Print '0,2'	0,2
4	0	3	True	False	Inner loop ends, increment i
5	1	0	True	True	Print '1,0'	1,0
6	1	1	True	True	Print '1,1'	1,1
7	1	2	True	True	Print '1,2'	1,2
8	1	3	True	False	Inner loop ends, increment i
9	2	0	False	N/A	Outer loop ends

💡 Outer loop i=2 is not less than 2, so the loop stops.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	After Step 8	Final
i	0	0	0	0	0	1	1	1	1	2
j	0	0	1	2	3	0	1	2	3	N/A

Key Moments - 3 Insights

Why does the inner loop run completely before the outer loop increments?

Because in the execution_table rows 1-4 and 5-8, you can see j goes from 0 to 3 fully for each i before i increments.

Why does the outer loop stop when i equals 2?

Row 9 shows condition i<2 is False, so the outer loop ends as per the exit_note.

What happens to variable j after the inner loop ends?

At steps 4 and 8, j reaches 3 which fails the inner loop condition, then resets to 0 when the inner loop starts again for the next i.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of j at Step 3?

A 3

B 2

C 1

D 0

💡 Hint

Check the 'j' column in execution_table row 3.

At which step does the outer loop condition become false?

A Step 9

B Step 4

C Step 8

D Step 1

💡 Hint

Look at the 'Condition i<2' column and exit_note in execution_table.

If the inner loop condition changed to j<2, how many times would 'Print' action happen in total?

A 8

B 6

C 4

D 3

💡 Hint

Multiply outer loop count (2) by new inner loop count (2) for total prints.

Concept Snapshot

Nested loops run one inside another.
Outer loop runs first.
Inner loop runs fully each outer iteration.
Inner loop resets each time outer loop increments.
Loops stop when their conditions fail.

Full Transcript

This example shows nested loops in PHP. The outer loop variable i starts at 0 and runs while less than 2. For each i, the inner loop variable j runs from 0 to less than 3. Each pair i,j is printed. The inner loop completes fully before the outer loop increments. When i reaches 2, the outer loop stops. Variables i and j change as shown in the variable tracker. Key points include the inner loop running fully each outer iteration and loops stopping when conditions fail.